Fix ind(D,d,x,y,p) in (i), (ii) and (iii) of Lemma 2.1.4

ind(D,d) : \Pi_{x,y : A} \Pi_{p : x = y} D(x,y,p) according to
Section 1.12.1 (Path induction).
Merge pull request #693 from ahorn/lemma-2.1.4

Fix ind(D,d,x,y,p) in (i), (ii) and (iii) of Lemma 2.1.4