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

### 5.4.5 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 the string s of size q beginning with the character at 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"