There are several ways to do this. The easiest is to add more grammar to the parser and let it do the hard work:

Object -> computer "computer"
  with name "computer",
       orders
       [;  Theta: print_ret "~Theta now set to ", noun, ".~";
           default: print_ret "~Please rephrase.~";
       ],
  has  talkable;
...
[ ThetaSub; "You must tell your computer so."; ];
Verb "theta" * "is" number -> Theta;