Mathlib v3 is deprecated. Go to Mathlib v4

Commit 2020-07-09 15:24 593a4c83

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fix(tactic/norm_cast): remove bad norm_cast lemma (#3340) This was identified as a move_cast lemma, which meant it was rewriting to the LHS which it couldn't reduce. It's better to let the conditional rewriting handle nat subtraction -- if the right inequality is in the context there's no need to go to int.sub_nat_nat.

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