`WF_INDUCT_TAC : term -> (string * thm) list * term -> goalstate`

SYNOPSIS
Performs wellfounded induction with respect to a given `measure'.

DESCRIPTION
The tactic WF_INDUCT_TAC is applied to two arguments. The second is a goal to prove, and the first is an expression to use as a ``measure''. The result is a new subgoal where the same goal is to be proved but as an assumption it holds for all smaller values of the measure, universally quantified over the free variables in the measure term (which should also be free in the goal).

FAILURE CONDITIONS
Never fails.

EXAMPLE
Suppose we define a Euclidean GCD algorithm:
```  # let egcd = define
`egcd(m,n) = if m = 0 then n
else if n = 0 then m
else if m <= n then egcd(m,n - m)
else egcd(m - n,n)`;;
```
and after picking up from the library an infix `divides' relation for divisibility:
```  # needs "Library/prime.ml";;
```
we want to prove something about the result, e.g.
```  # g `!m n d. d divides egcd(m,n) <=> d divides m /\ d divides n`;;
```
A natural way to proceed is by induction on the sum of the arguments:
```  # e(GEN_TAC THEN GEN_TAC THEN WF_INDUCT_TAC `m + n`);;
val it : goalstack = 1 subgoal (1 total)

0 [`!m'' n'.
m'' + n' < m + n
==> (!d. d divides egcd (m'',n') <=> d divides m'' /\ d divides n')`]

`!d. d divides egcd (m,n) <=> d divides m /\ d divides n`
```
Note that we have the same goal, but an assumption that it holds for smaller values of the measure term.