Given (H_1 : a = a), we have that
eq.rec H_2 H_1
reduces to H_2
This is not exclusive to equality.
It applies to any inductive datatype in Prop, containing only one
constructor with zero "arguments" (we say they are nullary).
BTW, the restriction to only one constructor is not needed, but it is
does not buy much to support multiple nullary constructors since Prop is
proof irrelevant.
