Meaning of stacked arrows

Question

I just stumbled across (pag 5) a symbol made of two arrows stacked together (\rightrightarrow in latex, it seems that MathJax does not recognize it).

The author uses it without giving a definition, so I guess it is supposed to be quite standard/well-known in the field but I never met it. May someone point me to the definition?

Answer 1

See : Klaus Weihrauch, Computable Analysis : An Introduction (2000), page 11:

Usually we will denote a correspondence $f$ from $A$ to $B$ by $f : \subseteq A \rightrightarrows B$. A partial function $f : \subseteq A \rightarrow B$ from $A$ to $B$ is a multi-valued fucntion $f : \subseteq A \rightrightarrows B$ such that the set $f[ \{ a \} ]$ contains only one element for each $a \in \text{dom}(f)$ .