  The positive part $U^+_q$ of $U_q({\widehat {\mathfrak{sl}}}_2)$ has a
presentation with two generators $A,B$ that satisfy the cubic $q$-Serre
relations. In 1993 I. Damiani obtained a PBW basis for
  $U^+_q$, consisting of some elements $\lbrace
E_{n\delta+\alpha_0}\rbrace_{n=0}^\infty$, $\lbrace
E_{n\delta+\alpha_1}\rbrace_{n=0}^\infty$, $\lbrace
E_{n\delta}\rbrace_{n=1}^\infty$ that are defined recursively. Our goal is to
describe these elements in closed form. We achieve this goal using Catalan
words and a $q$-shuffle algebra.
