When applied to a list of tactics [T1;...;Tn], and a goal g, the tactical
FIRST tries applying the tactics to the goal until one succeeds. If the
first tactic which succeeds is Tm, then the effect is the same as just Tm.
Thus FIRST effectively behaves as follows:
FIRST [T1;...;Tn] = T1 ORELSE ... ORELSE Tn