Let mixins only be arg-less ctors or objects.
Let mixins only be arg-less ctors or objects.

jacwright/substance,andene/substance,TypesetIO/substance,abhaga/substance,andene/substance,TypesetIO/substance,michael/substance-1,stencila/substance,jacwright/substance,substance/substance,michael/substance-1,substance/substance,abhaga/substance,podviaznikov/substance,podviaznikov/substance