@find(t:tab, f:function)
@find(m:map, f:function)
@find(s:string, f:function)
returns the index of the first element of t, m or s that satisfies the predicate f.

The predicate is a binary function taking the index or the key as the first argument and the associated value for the second argument.

The undef value (for maps) or the integer -1 (for tab and string) is returned if there is no pair satisfying the predicate.

See also @count, @member and @occurs.

Example: 

       @fun_def p(i, v) { return (i > 1) && (v % 2 == 0) }
       @find([1, 2, 3, 4], @p)  ; returns 4
@find returns the first index greather than one such that the corresponding element is a multiple of two.

       @fun_def p(i, v) { return (i > 1) && (v >= "b") }
       @find("abcdefg", @p)  ; returns 2
the index 2 is returned because the "c" is the first character greather or equal to "b" such that its position is strictly greather than one (character numbering starts at zero).