 
 
 
5.4.2  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 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" 
 
 
