When a code or data line is selected in the code list, if the operand
is an address inside the file, we highlight the address and label.
It's also useful to highlight the other way: when a code or data line
is selected, find all lines whose operands reference it, and highlight
the operand field.
This is a little trickier because there can be multiple references,
but all of the information we need is in the cross-reference table.
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