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
validated.
For example the Length entry in a StreamElement
is a number indicating the number of bytes of data in the
stream.
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
provided.
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.
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