@occurs(tab, value)
@occurs(map, value)
@occurs(string, value)
returns the first index or the first key whose value equals the second argument. For example 
       @occurs(["a", "b", "c", "a", "b"], "b") -> 1
       @occurs("xyz", "z")  -> 2
       @occurs(map{ ("zero", 0), ("null", 0), ("void", 0) }, 0)  -> "null"
In the last example, the answer "null" is returned because {"null" < "void" < "zero".

See also @count, @find, @is_prefix, @is_subsequence, @is_suffix and @member.

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