fix error in wlmetric_ball_serial

src/Groups.jl

@@ -72,7 +72,6 @@ word-length metric on the group generated by `S`. The ball is centered at `cente
 (by default: the identity element). `radius` and `op` keywords specify the
 radius and multiplication operation to be used.
 """
-
 function wlmetric_ball_serial(
     S::AbstractVector{T};
     radius = 2,
@@ -81,10 +80,7 @@ function wlmetric_ball_serial(
     old = unique!([one(first(S)), S...])
     sizes = [1, length(old)]
     for i = 2:radius
-        new = collect(
-            op(i, j)
-            for (i, j) in Base.product(@view(old[sizes[end-1]:end]), S)
-        )
+        new = collect(op(o, s) for o in @view(old[sizes[end-1]:end]) for s in S)
         append!(old, new)
         resize!(new, 0)
         old = unique!(old)

test/runtests.jl

@@ -15,6 +15,8 @@ using LinearAlgebra
       S = [w,r,s]; S = unique([S; inv.(S)]);
       _, sizes = Groups.wlmetric_ball(S, radius=4);
       @test sizes == [7, 33, 141, 561]
+      _, sizes = Groups.wlmetric_ball_serial(S, radius=4);
+      @test sizes == [7, 33, 141, 561]
    end
 
    include("FreeGroup-tests.jl")