When applied to a tactic-producing function f and an operand list
[x1,...,xn], the elements of which have the same type as f’s domain
type, MAP_FIRST maps the function f over the list, producing a list of
tactics, then tries applying these tactics to the goal till one succeeds.
If f(xm) is the first to succeed, then the overall effect is the same
as applying f(xm). Thus:
MAP_FIRST f [x1,...,xn] = (f x1) ORELSE ... ORELSE (f xn)