`CONV_TAC : conv -> tactic`

SYNOPSIS
Makes a tactic from a conversion.

DESCRIPTION
If c is a conversion, then CONV_TAC c is a tactic that applies c to the goal. That is, if c maps a term `g` to the theorem |- g = g', then the tactic CONV_TAC c reduces a goal g to the subgoal g'. More precisely, if c `g` returns A' |- g = g', then:
```         A ?- g
===============  CONV_TAC c
A ?- g'
```
In the special case where `g` is `T`, the call immediately solves the goal rather than generating a subgoal A ?- T. And in a slightly liberal interpretation of ``conversion'', the conversion may also just prove the goal and return A' |- g, in which case again the goal will be completely solved. Note that in all cases the conversion c should return a theorem whose assumptions are also among the assumptions of the goal (normally, the conversion will returns a theorem with no assumptions). CONV_TAC does not fail if this is not the case, but the resulting tactic will be invalid, so the theorem ultimately proved using this tactic will have more assumptions than those of the original goal.

FAILURE CONDITIONS
CONV_TAC c applied to a goal A ?- g fails if c fails when applied to the term g. The function returned by CONV_TAC c will also fail if the function c is not, in fact, a conversion (i.e. a function that maps a term t to a theorem |- t = t').

USES
CONV_TAC can be used to apply simplifications that can't be expressed as equations (rewrite rules). For example, a goal:
```  # g `abs(pi - &22 / &7) <= abs(&355 / &113 - &22 / &7)`;;
```
can be simplified by rational number arithmetic:
```  # e(CONV_TAC REAL_RAT_REDUCE_CONV);;
val it : goalstack = 1 subgoal (1 total)

`abs (pi - &22 / &7) <= &1 / &791`
```
It is also handy for invoking decision procedures that only have a ``rule'' form, and no special ``tactic'' form. (Indeed, the tactic form can be defined in terms of the rule form by using CONV_TAC.) For example, the goal:
```  # g `!x:real. &0 < x ==> &1 / x - &1 / (x + &1) = &1 / (x * (x + &1))`;;
```
can be solved by:
```  # e(CONV_TAC REAL_FIELD);;
...
val it : goalstack = No subgoals
```