? - Ternary If
The resulting link's arity is the same as the consumed link's arity. Note that the arity refers to that of the resulting link, not the number of links this quick consumes.
Ternary If; call a link depending on the result of a conditional. The resulting link's arity is the maximum of its consumed links' arities.
<if-link><else-link><condition>?- if the result of calling
<condition>is truthy, call
<if-link>; otherwise, call