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While DECHDL sequences should be used in pairs on adjacent lines, the first line drawing the top half, and the second line drawing the bottom half, apparently real VTxxx (or at least the VT220) terminals did draw only the top or bottom of characters when confronted with an isolated DECDHL line (https://retrocomputing.stackexchange.com/questions/18023/). xterm has that same behavior. So behave likewise.
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