# x ≤ m ⊦ (⌜x⌝ + σ1)[m] = σ1[m - # x]
Used simplifier rules: size-fin: ⊦ # ⌜x⌝ = ⌜# x⌝ ;% ⊦ m < n ↔ ¬ n ≤ m; (ss)% ⊦ ⌜m⌝ < ⌜n⌝ ↔ m < n; (s)% ⊦ ⌜m⌝ .n = m; (s)