[TYPES] What are congruence rules called?

[Norman Ramsey]

 > Question: Is there a standard greek letter for naming the congruence rules,
 > such as κ or γ? Or any other relevant naming convention?

In his January 2015 notes on lambda calculus, Prakash Panangaden
uses μ, ν, and ξ for congruence rules.  Perhaps he can comment
on sources?


Norman