XPath30 and XQuery30 section 3.2.2 "Dynamic Function Invocation" say:

    [Definition: A dynamic function invocation consists of a PrimaryExpr
    that returns the function item and a parenthesized list of zero or
    more arguments (argument expressions or ArgumentPlaceholders).]

    If the PrimaryExpr does not return a sequence consisting of a single
    function item with the same arity as the number of specified arguments,
    a type error is raised [err:XPTY0004].

These two occurrences of "PrimaryExpr" are problematic. Consider the section's second example:
    $f[2]("Hi there")
The function item that is invoked is returned by $f[2], which is not a PrimaryExpr. The PrimaryExpr is $f, which does not return a sequence consisting of a single function item.

I suggest we change "PrimaryExpr" to "base expression", in parallel with 3.2.1 "Filter Expressions".