#1157: Fix subtract function for Quantity.
#1157: Fix subtract function for Quantity.

lsaffre/lino,lino-framework/lino,lsaffre/lino,lino-framework/lino,lino-framework/lino,lsaffre/lino,lsaffre/lino,lino-framework/lino,lino-framework/lino,lsaffre/lino