A tentative to fix a bug in clock checking appearing in a program of

Timothy Bourke (trigerred where the clock of some args are some other
args and when the names of variables are shared between the caller
and the callee).

The problem is in UnifyClock.f or (in evalClock.ml) ; the current
change in evalClock.ml fixes the pb in Tim's program, but I suspect
it is still buggy.