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### `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

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