diff --git a/src/main/java/com/metaweb/gridworks/expr/ExpressionUtils.java b/src/main/java/com/metaweb/gridworks/expr/ExpressionUtils.java
index c1b973fc3..72485c4a9 100644
--- a/src/main/java/com/metaweb/gridworks/expr/ExpressionUtils.java
+++ b/src/main/java/com/metaweb/gridworks/expr/ExpressionUtils.java
@@ -91,6 +91,18 @@ public class ExpressionUtils {
             new EvalError(v.getClass().getSimpleName() + " value not storable");
     }
     
+    static public boolean isArray(Object v) {
+        return v != null && v.getClass().isArray();
+    }
+    
+    static public boolean isArrayOrCollection(Object v) {
+        return v != null && (v.getClass().isArray() || v instanceof Collection<?>);
+    }
+    
+    static public boolean isArrayOrList(Object v) {
+        return v != null && (v.getClass().isArray() || v instanceof List<?>);
+    }
+    
     @SuppressWarnings("unchecked")
     static public List<Object> toObjectList(Object v) {
         return (List<Object>) v;
diff --git a/src/main/java/com/metaweb/gridworks/gel/Control.java b/src/main/java/com/metaweb/gridworks/gel/Control.java
index 83598c0ae..4de85e3ad 100644
--- a/src/main/java/com/metaweb/gridworks/gel/Control.java
+++ b/src/main/java/com/metaweb/gridworks/gel/Control.java
@@ -5,6 +5,11 @@ import java.util.Properties;
 import com.metaweb.gridworks.Jsonizable;
 import com.metaweb.gridworks.expr.Evaluable;
 
+/**
+ * Interface of GEL controls such as if, forEach, forNonBlank, with. A control can
+ * decide which part of the code to execute and can affect the environment bindings.
+ * Functions, on the other hand, can't do either.
+ */
 public interface Control extends Jsonizable {
     public Object call(Properties bindings, Evaluable[] args);
     
diff --git a/src/main/java/com/metaweb/gridworks/gel/Function.java b/src/main/java/com/metaweb/gridworks/gel/Function.java
index 0bbafbfa2..a620e1f73 100644
--- a/src/main/java/com/metaweb/gridworks/gel/Function.java
+++ b/src/main/java/com/metaweb/gridworks/gel/Function.java
@@ -4,6 +4,10 @@ import java.util.Properties;
 
 import com.metaweb.gridworks.Jsonizable;
 
+/**
+ * Interface for functions. When a function is called, its arguments have already
+ * been evaluated down into non-error values.
+ */
 public interface Function extends Jsonizable {
     public Object call(Properties bindings, Object[] args);
 }
diff --git a/src/main/java/com/metaweb/gridworks/gel/Parser.java b/src/main/java/com/metaweb/gridworks/gel/Parser.java
index 4befab2e2..e0ed9ff70 100644
--- a/src/main/java/com/metaweb/gridworks/gel/Parser.java
+++ b/src/main/java/com/metaweb/gridworks/gel/Parser.java
@@ -18,7 +18,7 @@ import com.metaweb.gridworks.gel.ast.OperatorCallExpr;
 import com.metaweb.gridworks.gel.ast.VariableExpr;
 
 public class Parser {
-    protected Scanner     _scanner;
+    protected Scanner   _scanner;
     protected Token     _token;
     protected Evaluable _root;
     
@@ -47,6 +47,10 @@ public class Parser {
         return new ParsingException("Parsing error at offset " + index + ": " + desc);
     }
     
+    /**
+     *  <expression> := <sub-expression>
+     *                | <expression> [ "<" "<=" ">" ">=" "==" "!=" ] <sub-expression>
+     */
     protected Evaluable parseExpression() throws ParsingException {
         Evaluable sub = parseSubExpression();
         
@@ -66,6 +70,10 @@ public class Parser {
         return sub;
     }
     
+    /**
+     *  <sub-expression> := <term>
+     *                    | <sub-expression> [ "+" "-" ] <term>
+     */
     protected Evaluable parseSubExpression() throws ParsingException {
         Evaluable sub = parseTerm();
         
@@ -85,6 +93,10 @@ public class Parser {
         return sub;
     }
     
+    /**
+     *  <term> := <factor>
+     *          | <term> [ "*" "/" ] <factor>
+     */
     protected Evaluable parseTerm() throws ParsingException {
         Evaluable factor = parseFactor();
         
@@ -104,6 +116,17 @@ public class Parser {
         return factor;
     }
     
+    /**
+     *  <term> := <term-start> ( <path-segment> )* 
+     *  <term-start> :=
+     *      <string> | <number> | - <number> | <regex> | <identifier> |
+     *      <identifier> ( <expression-list> )
+     *      
+     *  <path-segment> := "[" <expression-list> "]"
+     *                  | "." <identifier>
+     *                  | "." <identifier> "(" <expression-list> ")"
+     *  
+     */
     protected Evaluable parseFactor() throws ParsingException {
         if (_token == null) {
             throw makeException("Expecting something more at end of expression");
@@ -214,6 +237,11 @@ public class Parser {
         return eval;
     }
     
+    /**
+     *  <expression-list> := <empty>
+     *                     | <expression> ( "," <expression> )*
+     *  
+     */
     protected List<Evaluable> parseExpressionList(String closingDelimiter) throws ParsingException {
         List<Evaluable> l = new LinkedList<Evaluable>();
         
diff --git a/src/main/java/com/metaweb/gridworks/gel/Scanner.java b/src/main/java/com/metaweb/gridworks/gel/Scanner.java
index 383767260..ea6f43909 100644
--- a/src/main/java/com/metaweb/gridworks/gel/Scanner.java
+++ b/src/main/java/com/metaweb/gridworks/gel/Scanner.java
@@ -14,8 +14,8 @@ public class Scanner {
     static public class Token {
         final public int        start;
         final public int        end;
-        final public TokenType    type;
-        final public String        text;
+        final public TokenType  type;
+        final public String     text;
         
         Token(int start, int end, TokenType type, String text) {
             this.start = start;
@@ -26,7 +26,7 @@ public class Scanner {
     }
     
     static public class ErrorToken extends Token {
-        final public String        detail; // error detail
+        final public String detail; // error detail
         
         public ErrorToken(int start, int end, String text, String detail) {
             super(start, end, TokenType.Error, text);
@@ -52,9 +52,9 @@ public class Scanner {
         }
     }
     
-    protected String     _text;
-    protected int        _index;
-    protected int        _limit;
+    protected String     _text;  // input text to tokenize
+    protected int        _index; // index of the next character to process
+    protected int        _limit; // process up to this index
     
     public Scanner(String s) {
         this(s, 0, s.length());
@@ -70,6 +70,16 @@ public class Scanner {
         return _index;
     }
     
+    /**
+     * The regexPossible flag is used by the parser to hint the scanner what to do
+     * when it encounters a slash. Since the divide operator / and the opening 
+     * delimiter of a regex literal are the same, but divide operators and regex
+     * literals can't occur at the same place in an expression, this flag is a cheap
+     * way to distinguish the two without having to look ahead.
+     * 
+     * @param regexPossible
+     * @return
+     */
     public Token next(boolean regexPossible) {
         // skip whitespace
         while (_index < _limit && Character.isWhitespace(_text.charAt(_index))) {
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/ControlCallExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/ControlCallExpr.java
index 0fcb38b45..08272854e 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/ControlCallExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/ControlCallExpr.java
@@ -5,6 +5,9 @@ import java.util.Properties;
 import com.metaweb.gridworks.expr.Evaluable;
 import com.metaweb.gridworks.gel.Control;
 
+/**
+ * An abstract syntax tree node encapsulating a control call, such as "if".
+ */
 public class ControlCallExpr implements Evaluable {
     final protected Evaluable[] _args;
     final protected Control     _control;
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/FieldAccessorExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/FieldAccessorExpr.java
index 586598a59..bb5a045da 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/FieldAccessorExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/FieldAccessorExpr.java
@@ -7,6 +7,11 @@ import com.metaweb.gridworks.expr.Evaluable;
 import com.metaweb.gridworks.expr.ExpressionUtils;
 import com.metaweb.gridworks.expr.HasFields;
 
+/**
+ * An abstract syntax tree node encapsulating a field accessor,
+ * e.g., "cell.value" is accessing the field named "value" on the
+ * variable called "cell".
+ */
 public class FieldAccessorExpr implements Evaluable {
     final protected Evaluable     _inner;
     final protected String        _fieldName;
@@ -19,7 +24,7 @@ public class FieldAccessorExpr implements Evaluable {
     public Object evaluate(Properties bindings) {
         Object o = _inner.evaluate(bindings);
         if (ExpressionUtils.isError(o)) {
-            return o;
+            return o; // bubble the error up
         } else if (o == null) {
             return new EvalError("Cannot retrieve field from null");
         } else if (o instanceof HasFields) {
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/FunctionCallExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/FunctionCallExpr.java
index 70f770bd1..058d71bf2 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/FunctionCallExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/FunctionCallExpr.java
@@ -6,6 +6,12 @@ import com.metaweb.gridworks.expr.Evaluable;
 import com.metaweb.gridworks.expr.ExpressionUtils;
 import com.metaweb.gridworks.gel.Function;
 
+/**
+ * An abstract syntax tree node encapsulating a function call. The function's
+ * arguments are all evaluated down to values before the function is applied.
+ * If any argument is an error, the function is not applied, and the error is
+ * the result of the expression.
+ */
 public class FunctionCallExpr implements Evaluable {
     final protected Evaluable[] _args;
     final protected Function    _function;
@@ -20,7 +26,7 @@ public class FunctionCallExpr implements Evaluable {
         for (int i = 0; i < _args.length; i++) {
             Object v = _args[i].evaluate(bindings);
             if (ExpressionUtils.isError(v)) {
-                return v;
+                return v; // bubble up the error
             }
              args[i] = v;
         }
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/LiteralExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/LiteralExpr.java
index 6c3b70ab3..34337ef23 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/LiteralExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/LiteralExpr.java
@@ -6,6 +6,9 @@ import org.json.JSONObject;
 
 import com.metaweb.gridworks.expr.Evaluable;
 
+/**
+ * An abstract syntax tree node encapsulating a literal value.
+ */
 public class LiteralExpr implements Evaluable {
     final protected Object _value;
     
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/OperatorCallExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/OperatorCallExpr.java
index 309e7caac..0731b1e41 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/OperatorCallExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/OperatorCallExpr.java
@@ -5,6 +5,9 @@ import java.util.Properties;
 import com.metaweb.gridworks.expr.Evaluable;
 import com.metaweb.gridworks.expr.ExpressionUtils;
 
+/**
+ * An abstract syntax tree node encapsulating an operator call, such as "+".
+ */
 public class OperatorCallExpr implements Evaluable {
     final protected Evaluable[] _args;
     final protected String        _op;
diff --git a/src/main/java/com/metaweb/gridworks/gel/ast/VariableExpr.java b/src/main/java/com/metaweb/gridworks/gel/ast/VariableExpr.java
index 2a05e7702..9fc574d59 100644
--- a/src/main/java/com/metaweb/gridworks/gel/ast/VariableExpr.java
+++ b/src/main/java/com/metaweb/gridworks/gel/ast/VariableExpr.java
@@ -4,6 +4,9 @@ import java.util.Properties;
 
 import com.metaweb.gridworks.expr.Evaluable;
 
+/**
+ * An abstract syntax tree node encapsulating the retrieval of a variable's content.
+ */
 public class VariableExpr implements Evaluable {
     final protected String _name;
     
diff --git a/src/main/java/com/metaweb/gridworks/gel/controls/ForEach.java b/src/main/java/com/metaweb/gridworks/gel/controls/ForEach.java
index fa4ff65cf..232ee820d 100644
--- a/src/main/java/com/metaweb/gridworks/gel/controls/ForEach.java
+++ b/src/main/java/com/metaweb/gridworks/gel/controls/ForEach.java
@@ -2,7 +2,6 @@ package com.metaweb.gridworks.gel.controls;
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Iterator;
 import java.util.List;
 import java.util.Properties;
 
@@ -21,7 +20,8 @@ public class ForEach implements Control {
         if (args.length != 3) {
             return ControlFunctionRegistry.getControlName(this) + " expects 3 arguments";
         } else if (!(args[1] instanceof VariableExpr)) {
-            return ControlFunctionRegistry.getControlName(this) + " expects second argument to be a variable name";
+            return ControlFunctionRegistry.getControlName(this) + 
+                " expects second argument to be a variable name";
         }
         return null;
     }
@@ -30,7 +30,7 @@ public class ForEach implements Control {
         Object o = args[0].evaluate(bindings);
         if (ExpressionUtils.isError(o)) {
             return o;
-        } else if (o == null || (!o.getClass().isArray() && !(o instanceof Iterable<?>))) {
+        } else if (!ExpressionUtils.isArrayOrCollection(o)) {
             return new EvalError("First argument to forEach is not an array");
         }
         
@@ -40,36 +40,36 @@ public class ForEach implements Control {
         try {
             List<Object> results = null;
             
-        	if (o.getClass().isArray()) {
-	            Object[] values = (Object[]) o;
-	    
-	            results = new ArrayList<Object>(values.length);
-	            for (Object v : values) {
-	                bindings.put(name, v);
-	                
-	                Object r = args[2].evaluate(bindings);
-	                
-	                results.add(r);
-	            }
-        	} else {
-	            results = o instanceof Collection<?> ?
-	            		new ArrayList<Object>(ExpressionUtils.toObjectCollection(o).size()) : 
-            			new ArrayList<Object>();
-	            
-        		Iterator<Object> i = ExpressionUtils.toObjectCollection(o).iterator();
-        		while (i.hasNext()) {
-        			Object v = i.next();
-        			
-	                bindings.put(name, v);
-	                
-	                Object r = args[2].evaluate(bindings);
-	                
-	                results.add(r);
-        		}
-        	}
-        	
+            if (o.getClass().isArray()) {
+                Object[] values = (Object[]) o;
+                
+                results = new ArrayList<Object>(values.length);
+                for (Object v : values) {
+                    bindings.put(name, v);
+                    
+                    Object r = args[2].evaluate(bindings);
+                    
+                    results.add(r);
+                }
+            } else {
+                Collection<Object> collection = ExpressionUtils.toObjectCollection(o);
+                
+                results = new ArrayList<Object>(collection.size());
+                
+                for (Object v : collection) {
+                    bindings.put(name, v);
+                    
+                    Object r = args[2].evaluate(bindings);
+                    
+                    results.add(r);
+                }
+            }
+            
             return results.toArray(); 
         } finally {
+            /*
+             *  Restore the old value bound to the variable, if any.
+             */
             if (oldValue != null) {
                 bindings.put(name, oldValue);
             } else {
diff --git a/src/main/java/com/metaweb/gridworks/gel/controls/ForNonBlank.java b/src/main/java/com/metaweb/gridworks/gel/controls/ForNonBlank.java
index 0f8572fdf..8416518d6 100644
--- a/src/main/java/com/metaweb/gridworks/gel/controls/ForNonBlank.java
+++ b/src/main/java/com/metaweb/gridworks/gel/controls/ForNonBlank.java
@@ -16,7 +16,8 @@ public class ForNonBlank implements Control {
         if (args.length != 4) {
             return ControlFunctionRegistry.getControlName(this) + " expects 4 arguments";
         } else if (!(args[1] instanceof VariableExpr)) {
-            return ControlFunctionRegistry.getControlName(this) + " expects second argument to be a variable name";
+            return ControlFunctionRegistry.getControlName(this) + 
+                " expects second argument to be a variable name";
         }
         return null;
     }
@@ -28,12 +29,15 @@ public class ForNonBlank implements Control {
         String name = ((VariableExpr) var).getName();
         
         if (ExpressionUtils.isNonBlankData(o)) {
-            Object oldValue = bindings.containsKey(name) ? bindings.get(name) : null;
+            Object oldValue = bindings.get(name);
             bindings.put(name, o);
             
             try {
                 return args[2].evaluate(bindings);
             } finally {
+                /*
+                 *  Restore the old value bound to the variable, if any.
+                 */
                 if (oldValue != null) {
                     bindings.put(name, oldValue);
                 } else {
diff --git a/src/main/java/com/metaweb/gridworks/gel/controls/If.java b/src/main/java/com/metaweb/gridworks/gel/controls/If.java
index b851d66d5..2c6468fee 100644
--- a/src/main/java/com/metaweb/gridworks/gel/controls/If.java
+++ b/src/main/java/com/metaweb/gridworks/gel/controls/If.java
@@ -21,7 +21,7 @@ public class If implements Control {
     public Object call(Properties bindings, Evaluable[] args) {
         Object o = args[0].evaluate(bindings);
         if (ExpressionUtils.isError(o)) {
-            return o;
+            return o; // bubble the error up
         } else if (ExpressionUtils.isTrue(o)) {
             return args[1].evaluate(bindings);
         } else {
diff --git a/src/main/java/com/metaweb/gridworks/gel/controls/IsTest.java b/src/main/java/com/metaweb/gridworks/gel/controls/IsTest.java
index 9bc5357f4..7b0c9cb26 100644
--- a/src/main/java/com/metaweb/gridworks/gel/controls/IsTest.java
+++ b/src/main/java/com/metaweb/gridworks/gel/controls/IsTest.java
@@ -27,10 +27,7 @@ abstract class IsTest implements Control {
         throws JSONException {
     
         writer.object();
-        writer.key("description"); writer.value(
-            "Evaluates expression o. If it is true, evaluates expression eTrue and returns the result. " +
-            "Otherwise, evaluates expression eFalse and returns that result instead."
-        );
+        writer.key("description"); writer.value(getDescription());
         writer.key("params"); writer.value("expression o");
         writer.key("returns"); writer.value("boolean");
         writer.endObject();
diff --git a/src/main/java/com/metaweb/gridworks/gel/controls/With.java b/src/main/java/com/metaweb/gridworks/gel/controls/With.java
index 8bd760f9f..269213055 100644
--- a/src/main/java/com/metaweb/gridworks/gel/controls/With.java
+++ b/src/main/java/com/metaweb/gridworks/gel/controls/With.java
@@ -15,7 +15,8 @@ public class With implements Control {
         if (args.length != 3) {
             return ControlFunctionRegistry.getControlName(this) + " expects 3 arguments";
         } else if (!(args[1] instanceof VariableExpr)) {
-            return ControlFunctionRegistry.getControlName(this) + " expects second argument to be a variable name";
+            return ControlFunctionRegistry.getControlName(this) + 
+                " expects second argument to be a variable name";
         }
         return null;
     }
@@ -34,6 +35,9 @@ public class With implements Control {
             
             return args[2].evaluate(bindings);
         } finally {
+            /*
+             *  Restore the old value bound to the variable, if any.
+             */
             if (oldValue != null) {
                 bindings.put(name, oldValue);
             } else {