show a = c, from H1 ⬝ H2 : a = c
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have e1 [visible] : a = b, from H1,
have e2 : a = c, from e1 ⬝ H2,
have e3 : c = a, from e2 ⁻¹,
have e4 [visible] : b = a, from e1 ⁻¹,
show b = c, from e1 ⁻¹ ⬝ e2 :
  b = c