Identifiers for sets, relabelling functions, agents or propositions, and formal parameters standing for these things, begin with an upper case letter A - Z.

The second and subsequent characters of an identifier may be:

• lower case letters a - z
• upper case letters A - Z
• the digits 0 - 9
• the characters ? ! _ ` ' -
• the characters # ^ except in SCCS action names, where these characters have a special meaning
• anything you like between a pair of % signs, at the very end of an identifier (this feature was added for use by Glenn Bruns' value-passing front end).

• eps and tau may not be used as action names or formal action parameters
• T and F may not be used as proposition identifiers or formal parameters to propositions.