`SUB_CONV : conv -> conv`

SYNOPSIS
Applies a conversion to the top-level subterms of a term.

DESCRIPTION
For any conversion c, the function returned by SUB_CONV c is a conversion that applies c to all the top-level subterms of a term. If the conversion c maps t to |- t = t', then SUB_CONV c maps an abstraction `\x. t` to the theorem:
```   |- (\x. t) = (\x. t')
```
That is, SUB_CONV c `\x. t` applies c to the body of the abstraction `\x. t`. If c is a conversion that maps `t1` to the theorem |- t1 = t1' and `t2` to the theorem |- t2 = t2', then the conversion SUB_CONV c maps an application `t1 t2` to the theorem:
```   |- (t1 t2) = (t1' t2')
```
That is, SUB_CONV c `t1 t2` applies c to the both the operator t1 and the operand t2 of the application `t1 t2`. Finally, for any conversion c, the function returned by SUB_CONV c acts as the identity conversion on variables and constants. That is, if `t` is a variable or constant, then SUB_CONV c `t` returns |- t = t.

FAILURE CONDITIONS
SUB_CONV c tm fails if tm is an abstraction `\x. t` and the conversion c fails when applied to t, or if tm is an application `t1 t2` and the conversion c fails when applied to either t1 or t2. The function returned by SUB_CONV c may also fail if the ML function c is not, in fact, a conversion (i.e. a function that maps a term t to a theorem |- t = t').