LotusScript equivalents for the @Middle

Function Middle(fullString As String, startString As String, endString As String)
'Script equivalent to @Middle
startPosition = Instr(fullString, startString)
startLen = Len(startString)
If startPosition > 0 And startString <> "" Then
endPosition = Instr( Right$(fullString, (Len(fullString) - startPosition)), endString)
If endPosition > 0 Then
Middle = Mid$(fullString, (startPosition + startLen), _
Instr( (startPosition + startLen), fullString, endString) - (startPosition+ startLen) )
Else
Middle = Mid$ (fullString, (Instr(fullString, startString) + Len(startString)) , Len(fullString))
End If
Else
Middle = ""
End If
End Function