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This reverts commit 838ed39a.

also compatible with Coq 8.4

Conflicts:
	CHANGES
	Version
	lib/coq/real/Trigonometry.v
	share/provers-detection-data.conf
	src/coq-tactic/why3tac.ml4
	src/transform/compute.ml
	src/transform/simplify_formula.ml

Andrei Paskevich authored Jan 10, 2016 and Guillaume Melquiond committed Feb 05, 2016

split the ppat_ghost field in program patterns into two distinct
conditions:
- ppat_ghost, indicating that the pattern starts as ghost,
  meaning that all variables in it are ghost, too;
- ppat_fail, meaning that the pattern contains a refutable
  ghost subpattern, which makes the match in the extracted code
  impossible, which makes the whole match expression ghost.

Until now, the two conditions were disjunctively combined, making
admissible the invalid pattern matching in bench/p/b-d/ghost4.mlw.