(c) If an accented strong vowel precedes a weak, they form a
diphthong. The diphthong is rarely dissolved, and is usually marked
with a diaresis, if dissolution takes place.
Ex. _Beso del aura, onda de luz_, p. 170, XV, l. 5.
(d) If an accented strong vowel follows a weak they may or may not
form a diphthong.