How can i prevent empty variables outputting numbers??

[mattmakins]

I am writing a language parser using prolog.

I am using possible variables for example:

Noun_phrase --> poss_deter (PD), poss_noun(PN).

poss_noun(N)-->n(N).
poss_noun(PN)-->[ ].

if a noun isnt being parsed then the poss_noun-->[ ] is used. However when outputting the noun phrase this displays a bunch of numbers for the poss_noun variable. I assume this is the memory location. Is there anyway of preventing this.

Thanks