Applies first tactic that succeeds in a list given by mapping a function over a
list.
DESCRIBE
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:
The application of MAP_FIRST to a function and tactic list fails iff
the function does when applied to any of the elements of the list. The
resulting tactic fails iff all the resulting tactics fail when
applied to the goal.