Support for non-positive integer keys

[funnell]

Hello,

I was trying out your library and noticed it didn't like integer keys <= 0.

  select type(key)                                                                                                                                                                       
  class is(key_base)                                                                                                                                                                     
    is_key_allowed = .true.                                                                                                                                                              
  type is(integer(I1P))                                                                                                                                                                  
    if (key>0) is_key_allowed = .true.                                                                                                                                                   
  type is(integer(I2P))                                                                                                                                                                  
    if (key>0) is_key_allowed = .true.                                                                                                                                                   
  type is(integer(I4P))                                                                                                                                                                  
    if (key>0) is_key_allowed = .true.                                                                                                                                                   
  type is(integer(I8P))                                                                                                                                                                  
    if (key>0) is_key_allowed = .true.                                                                                                                                                   
  type is(character(len=*))                                                                                                                                                              
    is_key_allowed = .true.                                                                                                                                                              
  endselect

Is there a reason why this is the case? Can it be changed?
Thanks!