NotNil returns the receiver element if it isn't nil; otherwise, it returns
an unparented element with an empty string tag. This function simplifies
the task of writing code to ignore not-found results from element queries.

ValidateInput forces all ReadFrom* methods to validate that the
provided input is composed of well-formed XML before processing it. If
invalid XML is detected, the ReadFrom* methods return an error. Because
this option requires the input to be processed twice, it incurs a
significant performance penalty.

When reading from a stream reader that consumes input as it reads, validation
would fail because it would attempt to read the stream twice.

It was previously go 1.13, but 1.13 does not support io.ReadAll, and
I don't want to go through the hoops to make it backward compatible.

These functions make it easier to traverse sibling elements sharing the
same parent element.