void add_action(str)
string str;

Set up a local functio "str" to be called when user input matches the 
command given by "add_verb()". "add_action()" must be called first.
Functions called by a player command will get the arguments as a string.
It must then return false if it was the wrong command, otherwise true.

If it was the wrong command, the parser will continue searching for another
command, until one returns true.

For example, there can be a wand and a rod. Both of these objects defines
add_verb("wave"). One of them will be rendomly called first, and it must
look at the argument, and match against "wand" or "rod" respectively.

See also add_verb()