We state in §18.2.3 2(d):

Return the value of the expression in the next-value attribute of Q, evaluating the expression with a singleton focus set to $event("node") and with a dynamic context that binds the variable whose name is the accumulator name to the value $value

The last phrase is nonsense. It should say "that binds the variable whose name is Q{}value to the value of the argument $value".

Since this is a formal definition we should perhaps define the static and dynamic context rather more pedantically. As a minimum, we should make it clear that the argument $event is NOT available directly.