[mkgmap-dev] set display name problem ( Error in style: Error: (lines:14): Unrecognised command 'display_name', Could not open style null )

I would like to set up display name with lines in my style-file like highway=tertiary { display_name '${name}' | 'tertiary' } [0x04 ........] however currently this is impossible. Could someone adapt mkgmap to treat display_name processing like "name" processing? The reason for this is that adding lengthier text to name clutters up the map, but display_name will not be printed out, but only given for turn instructions and on the tool tip - which is exactly what I would need.