There were two cases in which we called error
  * An InfixE with an operator epxression other than VarE or ConE
  * A comprehension with empty Stmts, ie CompE []

Crashing doesn't help much.  Now the library puts in the pretty
printed output a textual signal about what went wrong.

This addresses the crash in Trac #7235, although doesn't fix
the underlying cause, which remains shrouded in obscurity.