fix bisection algorithm

src/mesh.jl

@@ -215,12 +215,14 @@ function bisect!(mesh::HMesh, marked_cells::Set)
 	push!(mesh.cells, Tuple(replace(SVector(vertices(mesh, c1)), c1_vs[1] => vbisect)))
 	push!(mesh.levels, 0)
 	c3 = lastindex(mesh.cells)
+	@assert(length(setdiff(c1_vs, c1_vs[1])) == 2)
 	replace!(edgemap[NTuple{2}(setdiff(c1_vs, c1_vs[1]))], c1 => c3)
 
 	# take care to produce positively oriented cells
 	push!(mesh.cells, Tuple(replace(SVector(vertices(mesh, c1)), c1_vs[2] => vbisect)))
 	push!(mesh.levels, 0)
 	c4 = lastindex(mesh.cells)
+	@assert(length(setdiff(c1_vs, c1_vs[2])) == 2)
 	replace!(edgemap[NTuple{2}(setdiff(c1_vs, c1_vs[2]))], c1 => c4)
 
         # flip is correct
@@ -237,18 +239,20 @@ function bisect!(mesh::HMesh, marked_cells::Set)
 	    push!(mesh.cells, Tuple(replace(SVector(vertices(mesh, c2)), c1_vs[1] => vbisect))) # c1_vs is correct
 	    push!(mesh.levels, 0)
 	    c5 = lastindex(mesh.cells)
-	    replace!(edgemap[NTuple{2}(setdiff(c2_vs, c1_vs[1]))], c1 => c5)
+	    @assert(length(setdiff(c2_vs, c1_vs[1])) == 2)
+	    replace!(edgemap[NTuple{2}(setdiff(c2_vs, c1_vs[1]))], c2 => c5)
 
 	    # take care to produce positively oriented cells
 	    push!(mesh.cells, Tuple(replace(SVector(vertices(mesh, c2)), c1_vs[2] => vbisect))) # c1_vs is correct
 	    push!(mesh.levels, 0)
 	    c6 = lastindex(mesh.cells)
-	    replace!(edgemap[NTuple{2}(setdiff(c2_vs, c1_vs[2]))], c1 => c6)
+	    @assert(length(setdiff(c2_vs, c1_vs[2])) == 2)
+	    replace!(edgemap[NTuple{2}(setdiff(c2_vs, c1_vs[2]))], c2 => c6)
 
 	    # flip is correct
 	    push!(edgemap[(c1_vs[1], vbisect)], c6)
 	    push!(edgemap[(c1_vs[2], vbisect)], c5)
-	    edgemap[(c1_vs[3], vbisect)] = [c5, c6]
+	    edgemap[(c2_vs[3], vbisect)] = [c5, c6]
 
 	    mesh.levels[c2] += 1
 	    delete!(marked_cells, c2)