more explanation of fold in Poly
Merge remote-tracking branch 'upstream/ssr' into ssr-work
MoreCoq: Solved silly_ex.
MoreCoq: Solved apply_exercise1.
MoreCoq: Solved apply_with_exercise.
MoreCoq: Solved sillyex1.
MoreCoq: Solved sillyex2.
MoreCoq: Solved practice.
MoreCoq: Solved plus_n_n_injective.
MoreCoq: Solved eqnP_r.
MoreCoq: Solved eqnP_r_informal.
MoreCoq: Solved gen_dep_practice.
MoreCoq: Solved generalization_practice_more.
MoreCoq: Solved size_cat_cons.
MoreCoq: Solved size_cat_twice.
MoreCoq: Solved override_shadow.
MoreCoq: Solved combine_split.
MoreCoq: Solved bool_fn_applied_thrice.
MoreCoq: Solved override_same.
MoreCoq: Solved eqnC.