Set value in second combo box based on first combo box

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frmChooseIssuesDialog has two combo boxes--cboFirstIssue and
cboLastIssue

When user selects issue using cboFirstIssue, I'd like cboLastIssue to
get the same value if it has no value.

Also, if value in cboLastIssue is less than value in cboFirstIssue, I'd
like to display message box and not allow choice.

I'm doing this for a friend and it's been too long since I did these
things to remember how to even get started on this kind of deal. My
brain is going........

.