----------- Entities -------------
datafile
observation
sheet


----------- Attributes -----------
datafile       name           text	/id/mandatory
observation    value          memo	
observation    r              short0	/id/mandatory
observation    c              short0	/id/mandatory
sheet          name           text	/id/mandatory


----------- Relations -----------


RELATIONS INVOLVING ..... datafile
many, sheet, located_in, one/id, datafile


RELATIONS INVOLVING ..... observation
many, observation, obtained_from, one/id, sheet


RELATIONS INVOLVING ..... sheet
many, sheet, located_in, one/id, datafile
many, observation, obtained_from, one/id, sheet