When applied to a boolean value b and a string s, the call warn b s
prints out ``Warning: s'' and a following newline to the terminal if b is
true and otherwise does nothing.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# let n = 7;;
val n : int = 7
# warn (n <> 0) "Nonzero value";;
Warning: Nonzero value
val it : unit = ()