`(THEN) : tactic -> tactic -> tactic`

SYNOPSIS
Applies two tactics in sequence.

DESCRIPTION
If t1 and t2 are tactics, t1 THEN t2 is a tactic which applies t1 to a goal, then applies the tactic t2 to all the subgoals generated. If t1 solves the goal then t2 is never applied.

FAILURE CONDITIONS
The application of THEN to a pair of tactics never fails. The resulting tactic fails if t1 fails when applied to the goal, or if t2 does when applied to any of the resulting subgoals.

EXAMPLE
Suppose we want to prove the inbuilt theorem DELETE_INSERT ourselves:
```  # g `!x y. (x INSERT s) DELETE y =
if x = y then s DELETE y else x INSERT (s DELETE y)`;;
```
We may wish to perform a case-split using COND_CASES_TAC, but since variables in the if-then-else construct are bound, this is inapplicable. Thus we want to first strip off the universally quantified variables:
```  # e(REPEAT GEN_TAC);;
val it : goalstack = 1 subgoal (1 total)

`(x INSERT s) DELETE y =
(if x = y then s DELETE y else x INSERT (s DELETE y))`
```
and then apply COND_CASES_TAC:
```  # e COND_CASES_TAC;;
...
```
A quicker way (starting again from the initial goal) would be to combine the tactics using THEN:
```  # e(REPEAT GEN_TAC THEN COND_CASES_TAC);;
...
```

```   EQ_TAC THENL [ASM_REWRITE_TAC[]; ASM_REWRITE_TAC[]]
```   EQ_TAC THEN ASM_REWRITE_TAC[]