RATOR_CONV : (conv -> conv)
Applies a conversion to the operator of an application.
If c is a conversion that maps a term "t1" to the theorem |- t1 = t1', then the conversion RATOR_CONV c maps applications of the form "t1 t2" to theorems of the form:
   |- (t1 t2) = (t1' t2)
That is, RATOR_CONV c "t1 t2" applies c to the operand of the application "t1 t2".
RATOR_CONV c tm fails if tm is not an application or if tm has the form "t1 t2" but the conversion c fails when applied to the term t1. The function returned by RATOR_CONV c may also fail if the ML function c:term->thm is not, in fact, a conversion (i.e. a function that maps a term t to a theorem |- t = t').
- RATOR_CONV BETA_CONV (Term `(\x y. x + y) 1 2`);
> val it = |- (\x y. x + y)1 2 = (\y. 1 + y) 2 : thm

HOL  Kananaskis-14