f <= (not a and not b and not c and not d) or 
      (not a and not b and     c and not d) or 
      (not a and     b and not c and     d) or 
      (not a and     b and     c and     d) or 
      (    a and not b and     c and not d) or 
      (    a and     b and not c and     d) or 
      (    a and     b and     c and not d) or 
      (    a and     b and     c and     d);