feat(library/data/nat/wf): add nat.lt is well founded theorem
feat(emacs/lean-mode): bind "C-c C-k" to quail-show-key

 - Use this to see how to type a unicode character
fix(emacs/lean-syntax): change comment symbols to be considered as punctuations

Please read the documentation of "modify-syntax-entry":

"The first character of NEWENTRY should be one of the following:
  _           symbol constituent
  .           punctuation
"

Close: #306
feat(frontends/lean): add [whnf] modifier to eval command
refactor(library/logic/cast): move heq declaration to a separate module

heq is be needed for some automatically generated constructions.
So, we want it available with the least number of dependencies.
refactor(library/data/prod): break into pieces to reduce dependencies

prod is needed for some automatically generated constructions.
So, it is important it is loaded in the environment as early as possible.
feat(library/hott/trunc): prove that n-types are (n+1)-types.
feat(hott/trunc): clean up some theorems, prove some basic theorems
sorry bug
refactor(typeof): move typeof to general_notation
doc(algebra): update markdown files
style(library): rename set_category to discrete_category
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