Revert "Removed the ability to express xx(x):fixed parsing"

This reverts commit 0e1d738b817864dfb80cd83a5712adafd64405a0.
Fixed parens-grouped expr parsing

Generated ASTs include the form (ap expr (ap expr' expr'')), or
`x(xx)` (for instance); which would fail parsing, although they
shouldn't!

This fixes that.
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