\startuseMPgraphic{chapter thumb}
path chap_frame, sect_frame ; pair pos,a,b ; picture text ;
if \somenamedheadnumber{chapter}{current} > 0 : curr_chap := \somenamedheadnumber{chapter}{current} ; else : curr_chap := 1 ; fi; if \somenamedheadnumber{chapter}{last} > 0 : last_chap := \somenamedheadnumber{chapter}{last} ; else : last_chap := 1 ; fi; if \somenamedheadnumber{section}{current} > 0 : curr_sect := \somenamedheadnumber{section}{current} ; else: curr_sect := 1 ; fi; if \somenamedheadnumber{section}{last} > 0 : last_sect := \somenamedheadnumber{section}{last} ; else: last_sect := 1; fi;
skip := TextHeight * (curr_chap-1)/last_chap ;