ctxSwitch
	ctxTypeSwitch
	ctxFile
	ctxDecl
	ctxSpec
	ctxDefer
	ctxCall  // any function call other than ctxCall2
	ctxCall2 // function call whose result is assigned to two variables
	ctxSelector
)

// walk walks the AST x, calling visit(f, x, context) for each node.
func (f *File) walk(x interface{}, context astContext, visit func(*File, interface{}, astContext)) {

<p>
In assignments, each value must be <a href="#Assignability">assignable</a>
to the type of the operand to which it is assigned, with the following special cases:
</p>

<ol>
<li>
	Any typed value may be assigned to the blank identifier.
</li>

<li>
	If an untyped constant
	is assigned to a variable of interface type or the blank identifier,

<p>
In assignments, each value must be <a href="#Assignability">assignable</a>
to the type of the operand to which it is assigned, with the following special cases:
</p>

<ol>
<li>
	Any typed value may be assigned to the blank identifier.
</li>

<li>
	If an untyped constant
	is assigned to a variable of interface type or the blank identifier,

		return ErrFormat
	}

	return nil
}

func readDataDescriptor(r io.Reader, f *File) error {
	var buf [dataDescriptorLen]byte
	// The spec says: "Although not originally assigned a
	// signature, the value 0x08074b50 has commonly been adopted
	// as a signature value for the data descriptor record.
	// Implementers should be aware that ZIP files may be