`SUBST_OCCS_TAC : (int list * thm) list -> tactic`
STRUCTURE
SYNOPSIS
Makes substitutions in a goal at specific occurrences of a term, using a list of theorems.
DESCRIPTION
Given a list (l1,A1|-t1=u1),...,(ln,An|-tn=un) and a goal (A,t), SUBST_OCCS_TAC replaces each ti in t with ui, simultaneously, at the occurrences specified by the integers in the list li = [o1,...,ok] for each theorem Ai|-ti=ui.
```              A ?- t
=============================  SUBST_OCCS_TAC [(l1,A1|-t1=u1),...,
A ?- t[u1,...,un/t1,...,tn]                   (ln,An|-tn=un)]
```
The assumptions of the theorems used to substitute with are not added to the assumptions A of the goal, but they are recorded in the proof. If any Ai is not a subset of A (up to alpha-conversion), SUBST_OCCS_TAC [(l1,A1|-t1=u1),...,(ln,An|-tn=un)] results in an invalid tactic.

SUBST_OCCS_TAC automatically renames bound variables to prevent free variables in ui becoming bound after substitution.

FAILURE
SUBST_OCCS_TAC [(l1,th1),...,(ln,thn)] (A,t) fails if the conclusion of any theorem in the list is not an equation. No change is made to the goal if the supplied occurrences li of the left-hand side of the conclusion of thi do not appear in t.
EXAMPLE
When trying to solve the goal
```   ?- (m + n) + (n + m) = (m + n) + (m + n)
```
applying the commutative law for addition on the third occurrence of the subterm m + n
```   SUBST_OCCS_TAC [([3], SPECL [Term `m:num`, Term `n:num`]
```   ?- (m + n) + (n + m) = (m + n) + (n + m)