Called to indicate that a particular RefAtom has been seen.
Most of the time the Start method can
be used to keep track of which IndirectObjects have been seen.
However occasionally a RefAtom may not resolve to an Element that can be
For example the Length entry in a StreamElement
is a number indicating the number of bytes of data in the
In most cases this is a simple number embedded as the value of
the Length entry in the dictionary.
However some PDFs represent this as a reference to an
IndirectObject containing a number.
Since it is a reference to a number there is no Element type for it and it
will not get validated.
For the basic validation process this will not matter since the
reference is automatically resolved and a simple number
However if you are keeping track of which IndirectObjects have
been validated, it is important to know about these cases.
You can do so by overriding this function.