C(9,7,4) <=6

Method of construction: lex covering

 1  2  3  4  5  6  7
 1  2  3  4  5  8  9
 1  2  3  6  7  8  9
 1  4  5  6  7  8  9
 2  3  4  5  6  7  8
 2  3  4  5  6  7  9