INT OK
add 1 OK
add 2 OK
print 
  1: 1
  2: _ 2
add 0 OK
add 4 OK
add 0 Error
add 4 Error
print 
  1: 1
  2: 0 2
  3: _ 4
STRING OK
add dog OK
add cat OK
add pig OK
add horse OK
print 
  1: dog
  2: cat pig
  3: horse _
add cat Error
add horse Error
print 
  1: dog
  2: cat pig
  3: horse _