--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| The HELM team. *)
+(* ||A|| http://helm.cs.unibo.it *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU General Public License Version 2 *)
+(* *)
+(**************************************************************************)
+
+include "basic_2/rt_transition/cpr.ma".
+include "apps_2/models/veq_lifts.ma".
+include "apps_2/models/deq.ma".
+
+(* DENOTATIONAL EQUIVALENCE ************************************************)
+
+(* Forward lemmas with context-sensitive parallel reduction for terms *******)
+
+lemma cpr_fwd_deq (h) (M): is_model M → is_extensional M →
+ ∀G,L,T1,T2. ⦃G, L⦄ ⊢ T1 ➡[h] T2 → ⦃G, L⦄ ⊢ T1 ≗{M} T2.
+#h #M #H1M #H2M #G #L #T1 #T2 #H @(cpr_ind … H) -G -L -T1 -T2
+[ /2 width=2 by deq_refl/
+| #G #K #V1 #V2 #W2 #_ #IH #HVW2 #gv #v #H
+ elim (li_inv_abbr … H) -H #lv #d #HK #Hd #H
+ @(mr … H1M) [4,5: @(ti_ext_l … H1M … H) |1,2: skip ] -v
+ lapply (lifts_SO_fwd_vlift … gv H1M H2M … HVW2 lv d) -HVW2 #HVW2
+ @(seq_trans … H1M … HVW2) -W2
+ @(seq_trans … H1M) [3: @IH // | skip ] -G -K -V2
+ @(seq_canc_dx … H1M … Hd) -V1 /2 width=1 by ti_lref_vlift_eq/
+| #I #G #K #T #U #i #_ #IH #HTU #gv #v #H
+ elim (li_fwd_bind … H) -H #lv #d #HK #H
+ @(mr … H1M) [4,5: @(ti_ext_l … H1M … H) |1,2: skip ] -v
+ lapply (lifts_SO_fwd_vlift … gv H1M H2M … HTU lv d) -HTU #HTU
+ @(seq_trans … H1M … HTU) -U
+ @(seq_trans … H1M) [3: @IH // | skip ] -G -K -T
+ /2 width=1 by ti_lref_vlift_gt/
+| #p * #G #L #V1 #V2 #T1 #T2 #_ #_ #IHV #IHT #gv #lv #Hlv
+ [ @(mr … H1M) [4,5: @(seq_sym … H1M) @(md … H1M) |1,2: skip ] -p
+ @(seq_trans … H1M) [3: @IHT /3 width=1 by li_abbr/ | skip ] -T2
+ /4 width=1 by ti_comp_l, veq_refl, vlift_comp/
+ | @(mx … H2M) /3 width=1 by li_abst/
+ ]
+| * #G #L #V1 #V2 #T1 #T2 #_ #_ #IHV #IHT #gv #lv #Hlv
+ [ @(mr … H1M) [4,5: @(seq_sym … H1M) @(ma … H1M) |1,2: skip ]
+ /3 width=1 by mc/
+ | @(mr … H1M) [4,5: @(seq_sym … H1M) @(me … H1M) |1,2: skip ]
+ /2 width=1 by/
+ ]
+| #G #L #V #U1 #U2 #T2 #_ #IH #HTU2 #gv #lv #Hlv
+ @(seq_trans … H1M) [2: @(md … H1M) | skip ]
+ @(seq_trans … H1M) [2: @IH /3 width=1 by li_abbr, veq_refl/ | skip ] -G -L -U1
+ /3 width=1 by lifts_SO_fwd_vlift, seq_sym/
+| #G #L #V #T1 #T2 #_ #IH #gv #lv #Hlv
+ @(seq_trans … H1M) [2: @(me … H1M) | skip ]
+ /2 width=1 by/
+| #p #G #L #V1 #V2 #W1 #W2 #T1 #T2 #_ #_ #_ #IHV #_ #IHT #gv #lv #Hlv
+ @(mr … H1M) [4,5: @(seq_sym … H1M) [ @(ma … H1M) | @(md … H1M) ] |1,2: skip ]
+ @(seq_trans … H1M) [3: @IHT /2 width=1 by li_abst/ | skip ] -T2
+ @(mr … H1M) [4,5: @(seq_sym … H1M) [ @(mb … H1M) | @(ti_comp_l … H1M) ] |1,2: skip ]
+ [2: @vlift_comp [2: @(me … H1M) |4: @(veq_refl … H1M) |1,3: skip ] | skip ]
+ /4 width=1 by ti_comp_l, veq_refl, vlift_comp/
+| #p #G #L #V1 #V #V2 #W1 #W2 #T1 #T2 #_ #_ #_ #IHV #IHW #IHT #HV2 #gv #lv #Hlv
+ @(mr … H1M) [4,5: @(seq_sym … H1M) [ @(ma … H1M) | @(md … H1M) ] |1,2: skip ]
+ @(mr … H1M) [4,5: @(seq_sym … H1M) [ @(mc … H1M) | @(ma … H1M) ] |1,2: skip ]
+ [2: @IHV // |4: @(md … H1M) |1,3: skip ] -p -V1
+ @(mc … H1M) [ /2 width=1 by lifts_SO_fwd_vlift/ ] -V -V2
+ @(seq_trans … H1M) [2: @IHT /3 width=1 by li_abbr, veq_refl/ | skip ] -T1
+ /4 width=1 by ti_comp_l, veq_refl, vlift_comp/
+]
+qed-.