Option
Monad
TODO: remove `OptionM` after update stage0 see: https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Do.20we.20still.20need.20OptionM.3F/near/279761084
tryPureCoe?
Based on the discussion at https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/for.2C.20unexpected.20need.20for.20type.20ascription/near/269083574 The consensus seemed to be that "auto pure" is more confusing than its worth.