my $trie = maketree();
extend-trie( $trie, 'fred', 10 );
extend-trie( $trie, 'bill', 20 );

## Does your lisp stuff define a way of iterating the keys?
my $temp = sum lookup( $trie, 'fred' ), lookup( $trie, 'bill' ); 
extend-trie( $trie, 'total', $temp );