two improvements of the guard condition: - a matched expression is reduced (in order to check if it's a subterm)   to hnf only when it contains variables that are subterms; - a matched expression is checked to be a subterm only when it belongs to   a *recursive* type.
two improvements of the guard condition:
- a matched expression is reduced (in order to check if it's a subterm)
  to hnf only when it contains variables that are subterms;
- a matched expression is checked to be a subterm only when it belongs to
  a *recursive* type.


git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@12553 85f007b7-540e-0410-9357-904b9bb8a0f7

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