`mp_then : match_position -> thm_tactic -> thm -> thm -> tactic`
STRUCTURE
SYNOPSIS
Matches two theorems against each other and then continues
DESCRIPTION
The mp_then tactic combines two theorems (one or both of which will typically come from the current goal’s assumptions) using modus ponens in a controlled way, and then passes the result of this to a continuation theorem-tactic.

Thus mp_then ttac pos ith th is a tactic in the usual “_then” fashion which produces a theorem and then applies the ttac continuation to that result. The theorems ith and th are the two theorems: ith contains the implication, and the other has a conclusion matching one of the antecedents of the implication.

An implication’s antecedents are calculated by first normalising the implication so that it is of the form

```  !v1 .. vn. ant1 /\ ant2 .. /\ antn ==> concl
```

The pos parameter indicates how to find the antecedent. There are four possible forms for pos (constructors for the match_position type). It can be Any, which tells mp_then to search for anything that works. It can be Pat q, with q a quotation, which means to find anything matching q that works. It can be Pos f, where f is a function of type term list -> term, and is typically a value such as hd, el n for some number n or last. This function is passed the list of all ith’s antecedents to pick the right one. Finally, the pos parameter might be Concl, meaning that the conclusion of ith is treated as a negated assumption. This allows implications to be used in a contrapositive way.

In practice, there are some common patterns for obtaining ith and th. Apart from the fully applied version above, you might also see:

```   <sel>_assum (mp_then pos ttac ith)

<sel>_assum (<sel>_assum o mp_then pos ttac)

disch_then(<sel>_assum o mp_then pos ttac)
```
where <sel> is one of first, last, qpat and goal, possibly with an appended _x; the usual ways of obtaining theorems from the current goal. Where there are two selectors used, the outermost is used for the selection of the implicational theorem and the innermost selects th. In the first example, the ith value is provided in the call, and is presumably an existing theorem rather than an assumption from the goal.

The goal_assum selector is worth special mention since it’s especially useful with mp_then: it turns an existentially quantified goal ?x. P x into the assumption !x. P x ==> F thereby providing a theorem with antecedents to match on. In conjunction with mp_tac (which will put the revised implication back into the goal as an existential once more) it has the effect of instantiating the existential quantifier in order to match a provided subterm (similar to part_match_exists_tac or asm_exists_tac).

Finally, note that the Pat and Any position selectors will backtrack across the set of theorem antecedents to find a match that makes the whole application succeed.

FAILURE
If the provided implicational theorem doesn’t have a match at a compatible position for the second provided theorem, or if no such match then allows the continuation ttac to succeed.
SEEALSO