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"