add ProductNotDefined error

src/mstructures.jl

@@ -13,6 +13,21 @@ _product(::Val{true}, g, h) = star(g) * h
 # additionally it may implement
 # * basis(ms)
 
+struct ProductNotDefined <: Exception
+    i::Any
+    j::Any
+    msg::Any
+end
+
+function Base.showerror(io::IO, ex::ProductNotDefined)
+    print(io, "Product of elements $(ex.i) and $(ex.j) is not defined on the basis")
+    print(io, " or the multiplicative structure could not be completed")
+    if isdefined(ex, :msg)
+        print(io, ": $(ex.msg)")
+    end
+    print(io, ".")
+end
+
 struct TrivialMStructure{Tw,I,B<:AbstractBasis} <: MultiplicativeStructure{Tw,I}
     basis::B
 end
@@ -28,5 +43,6 @@ Base.@propagate_inbounds function Base.getindex(mstr::TrivialMStructure, i::Inte
     b = basis(mstr)
     g, h = b[i], b[j]
     gh = _product(mstr, g, h)
+    gh in b || throw(ProductNotDefined(i, j, "$g · $h = $gh"))
     return b[gh]
 end