UNWIND_ALL_BUT_RIGHT_RULE : (string list -> thm -> thm)
    A |- !z1 ... zr.
          t =
          (?l1 ... lp. t1  /\ ... /\ eqn1 /\ ... /\ eqnm /\ ... /\ tn)
   ---------------------------------------------------------------------
    A |- !z1 ... zr.
          t =
          (?l1 ... lp. t1' /\ ... /\ eqn1 /\ ... /\ eqnm /\ ... /\ tn')
#UNWIND_ALL_BUT_RIGHT_RULE [`l2`]
# (ASSUME
#   "!f. IMP(f) =
#     ?l2 l1.
#      (!(x:num). l1 x = (l2 x) - 1) /\
#      (!x. f x = (l2 (x+1)) + (l1 (x+2))) /\
#      (!x. l2 x = 7)");;
. |- !f.
      IMP f =
      (?l2 l1.
        (!x. l1 x = (l2 x) - 1) /\
        (!x. f x = (l2(x + 1)) + ((l2(x + 2)) - 1)) /\
        (!x. l2 x = 7))