RemoveSwapsRewriter may fail if diagram is not normalised #215
Description
Correct process:
# d = some pregroup diagram
rewriter = RemoveSwapsRewriter()
d_new = rewriter(d.pregroup_normal_form()){{ message }}
Correct process:
# d = some pregroup diagram
rewriter = RemoveSwapsRewriter()
d_new = rewriter(d.pregroup_normal_form())