How to limit field to one line?

[kee nethery]

returnInField in case someone enters a return in the field.
closeField in case someone pastes multiple lines of text into the field

on returnInField
   put line 1 of me into me
   answer "Only one line of text is allowed in this field." with :OK"
end returnInField

on closeField
   if return is in me then
     returnInField
   end if
   pass closefield
end closefield