diffhistFormal language10:32+201Nyngwangtalkcontribs(Formalize the definition of "well-formed" by set theory; notice that this edit also formalized the possibly informal term "expression".)
diffhistFormal language10:00+54Nyngwangtalkcontribs(Prefer introducing the parent set Σ* first instead of doing so for its subset, since a reader should be able to derive the informal meaning of its subset themselves; use markdown tags for math symbols.)