Beta-reduces a top-level beta-redex on the right-hand side of an equation.
DESCRIBE
When applied to an equational theorem, RIGHT_BETA applies beta-reduction at
top level to the right-hand side (only). Variables are renamed if necessary to
avoid free variable capture.
A |- s = (\x. t1) t2
---------------------- RIGHT_BETA
A |- s = t1[t2/x]
FAILURE
Fails unless the theorem is equational, with its right-hand side being
a top-level beta-redex.