@is_prefix(s1:string, s2:string)
@is_prefix(s1:string, s2:string, cmp:fct)
@is_prefix(t1:tab, t2:tab)
@is_prefix(t1:tab, t2:tab, cmp:fct)
@is_prefix is an overloaded function. See also @is_suffix and @is_subsequence.

@is_prefix(s1:string, s2:string)
returs true if string s1is a prefix of s2.

@is_prefix(s1:string, s2:string, cmp:fct)
the predicate returns true if s1 is a prefix of s2 where the characters are compared with the function cmp (taking two arguments). The characters are passed to the function cmp as strings of length one.

@is_prefix(t1:tab, t2:tab)
the predicate returns true if t1 is a prefix of t2, that is, if the elements of t1 are the initial elements of t2.

@is_prefix(t1:tab, t2:tab, cmp:fct)
same as the previous version but the function is used to test the equality between the elements, instead of the usual comparison between values. For example: 
       @fun_def cmp($x, $y) { $x > $y }
       @is_prefix([11, 22], [5, 6, 7], @cmp) -> true
true is returned because @cmp(11, 6) and @cmp(22, 7) hold.

See also Predicates @!=,    @&&,    @||,    @<=,    @<,    @==,    @>=,    @>,    @approx,    @arch_darwin,    @arch_linux,    @arch_windows,    @between,    @empty,    @find,    @is_bool,    @is_defined,    @is_exec,    @is_fct,    @is_float,    @is_function,    @is_int,    @is_integer_indexed,    @is_list,    @is_map,    @is_nim,    @is_numeric,    @is_obj,    @is_obj_xxx,    @is_prefix,    @is_string,    @is_subsequence,    @is_suffix,    @is_symbol,    @is_undef,    @is_vector,    @member,    @occurs