How many (possibly empty) sets of lattice points {P1,P2,...,PM}, where each point Pi=(xi,yi) for xi,yi∈{0,1,2,3,4,5,6}, satisfy that the slope of the line PiPj is positive for each 1≤i<j≤M ? An infinite slope, e.g. Pi is vertically above Pj , does not count as positive.