`str_len', `str_chr', `sub_str'
-------------------------------

str_len(STR)
     :: Returns the length of a string.

str_chr(STR,START,C)
     :: Returns the position of the first occurrence of a character in
     a string.

sub_str(STR,START,END)
     :: Returns a substring of a string.

RETURN
     `str_len()', `str_chr()':integer; `sub_str()':string

STR C
     string

START END
     non-negative integer

   * `str_len()' returns the length of a string.

   * `str_chr()' scans a string STR from the START-th character and
     returns the position of the first occurrence of the first
     character of a string C. Note that the top of a string is the 0-th
     charater. It returns -1 if the character does not appear.

   * `sub_str()' generates a substring of STR containing characters
     from the START-th one to the END-th one.

     [185] Line="123 456 (x+y)^3";
     123 456 (x+y)^3
     [186] Sp1 = str_chr(Line,0," ");
     3
     [187] D0 = eval_str(sub_str(Line,0,Sp1-1));
     123
     [188] Sp2 = str_chr(Line,Sp1+1," ");
     7
     [189] D1 = eval_str(sub_str(Line,Sp1+1,Sp2-1));
     456
     [190] C = eval_str(sub_str(Line,Sp2+1,str_len(Line)-1));
     x^3+3*y*x^2+3*y^2*x+y^3

