INST : (term,term) subst -> thm -> thm
             A |- t               INST [x1 |-> t1,...,xn |-> tn]
   -----------------------------
    A[t1,...,tn/x1,...,xn] 
     |- 
    t[t1,...,tn/x1,...,xn]
  
   - load"arithmeticTheory";
   - CONJUNCT1 arithmeticTheory.ADD_CLAUSES;
   > val it = |- 0 + m = m : thm
   - INST [``m:num`` |-> ``2*x``] 
          (CONJUNCT1 arithmeticTheory.ADD_CLAUSES);
   val it = |- 0 + (2 * x) = 2 * x : thm