First character, middle and end of a string : head mid tail

### 2.4.2 First character, middle and end of a string : `head mid tail`

- `head(s)` returns the first character of the string `s`.
  Input :
  `head("Hello")`
  Output :
  `"H"`
- `mid(s,p,q)` returns the part of size `q` of the string `s` begining at the character of index `p`.
  Remember that the first index is 0 in Xcas mode.
  Input :
  `mid("Hello",1,3)`
  Output :
  `"ell"`
- `tail(s)` returns the string `s` without its first character.
  Input :
  `tail("Hello")`
  Output :
  `"ello"`