m-lines, called ‘movement lines’, is a fewest-line set reduced from an all-line map by using a greedy algorithm of tackling an NP-hard problem. It first selects a line which crosses the maximum number of s-lines, and then selects the line crosssing the second maximum number of s-lines (not crossed by the first line), and added to the set. And so on, until no more s-lines remain.
Sources Peponis, J., Wineman, J., Rashid, M., Kim, S. and Bafna, S. (1997), On the Generation of Linear Representation of Spatial Configuration. In: Proceedings of 1st Space Syntax Symposium, 41.01-18. pp. 41.03-05