`PROP_ATOM_CONV : conv -> conv`

SYNOPSIS
Applies a conversion to the `atomic subformulas' of a formula.

DESCRIPTION
When applied to a Boolean term, PROP_ATOM_CONV conv descends recursively through any number of the core propositional connectives `~', `/\', `\/', `==>' and `<=>', as well as the quantifiers `!x. p[x]', `?x. p[x]' and `?!x. p[x]'. When it reaches a subterm that can no longer be decomposed into any of those items (e.g. the starting term if it is not of Boolean type), the conversion conv is tried, with a reflexive theorem returned in case of failure. That is, the conversion is applied to the ``atomic subformulas'' in the usual sense of first-order logic.

FAILURE CONDITIONS
Never fails.

EXAMPLE
Here we swap all equations in a formula, but not any logical equivalences that are part of its logical structure:
``` # PROP_ATOM_CONV(ONCE_DEPTH_CONV SYM_CONV)
`(!x. x = y ==> x = z) <=> (y = z <=> 1 + z = z + 1)`;;
val it : thm =
|- ((!x. x = y ==> x = z) <=> y = z <=> 1 + z = z + 1) <=>
(!x. y = x ==> z = x) <=>
z = y <=>
z + 1 = 1 + z
```
By contrast, just ONCE_DEPTH_CONV SYM_CONV would just swap the top-level logical equivalence.

USES
Carefully constraining the application of conversions.