This program implements the login command. The 3 inputs it requires 
are :  - Login name
       - Password
       - Security level
user(raj,r1,1).
user(tom,t1,2).
user(jerry,j1,3).

checkusername(_loginname,_loginname,_count)
  :- user(_loginname,_,_).
checkusername(_candloginname,_loginname,_count)
  :- _count < 3,
     _newcount is 1 + _count,
     print('Incorrect loginname. Please reenter : '),
     read(_response),
     checkusername(_response,_loginname,_newcount).
checkusername(_candloginname,_loginname,_count)
  :- _count = 3,
     print('Sorry . Good Bye. '),
     false.

checkuserpassword(_loginname,_password,_password,_count)
  :- user(_loginname,_password,_).
checkuserpassword(_loginname,_candpassword,_password,_count)
  :- _count < 2,
     _newcount is 1 + _count,
     print('Incorrect Password. Please reenter : '),
     read(_response),
     checkuserpassword(_loginname,_response,_password,_newcount).
checkuserpassword(_loginname,_candpassword,_password,_count)
  :- _count = 2,
     print('Incorrect password. Start login again.'),
     fail.

checkusersecuritylevel(_loginname,_password,_securitylevel,_securitylevel,_count)
  :- user(_loginname,_password,_slevel),
     _securitylevel =< _slevel,
     print(('Create Session: ',_loginname,_password,_securitylevel)).
checkusersecuritylevel(_loginname,_password,_candsecuritylevel,_securitylevel,_count)
  :- _count < 3,
     _newcount is 1 + _count,
     print('Security level too high. Reenter it :'),
     read(_response),
     checkusersecuritylevel(_loginname,_password,_candsecuritylevel,_securitylevel,_newcount).
checkusersecuritylevel(_loginname,_password,_candsecuritylevel,securitylevel,_count)
  :- _count = 3,
     print(('Create Session: ',_loginname,_password,' Unclassified.')).
login :- print('Login name :'),
         read(_candloginname),
         checkusername(_candloginname,_loginname,1),
         print('Enter password :'),
         read(_candpassword),!,
         checkuserpassword(_loginname,_candpassword,_password,1),
         print('Enter security level :'),
         read(_candsecuritylevel),
         checkusersecuritylevel(_loginname,_password,_candsecuritylevel,_securitylevel,1).