| /3 width=2 by ex_intro, or_introl/
]
| -IH @or_introl @(ex_intro … 1) // (**) (* auto fails *)
- | lapply (transitive_lt s1 ??? Hs21) [ /2 width=1 by next_lt/ ] -Hs12 #Hs12
+ | lapply (transitive_lt s1 ??? Hs21) [ /2 width=1 by next_lt/ ] -Hs12 #Hs12
elim (IH (s2-⫯[h]s1)) -IH
[3: /3 width=1 by next_lt, monotonic_lt_minus_r/ ]
>minus_minus_m_m [2,4: /2 width=1 by lt_to_le/ ] -Hs21