Dynamically add tokens

[Alex Duller]

Hi,
In the language that I'm parsing it's possible to define custom 
'types';  I have the following two rules:

type_defn:
yTYPE yID yIS var_type ';'

use_type:
yID "=>" sig_type

So, I want to somehow store whatever the user enters as yID in the first 
rule and use it as an alias for the specified var_type when yID is used 
in the second rule.

Hope that makes sense :)

thanks,
Alex