Left panel: curves on which the x component (red) and the y component (blue) of the combined velocity field of four sources (filled gray circles) vanish. Null points (filled yellow circles) are identified as those locations where both lines meet outside of any source. Straight dashed lines connect those initial centers c (crosses) of a rectangular cell that did eventually converge onto a null to this null (which apparently need not be the null closest to c’s initial position). We also note that these dashed lines merely serve to illustrate which starting point converges upon which null, while convergence itself usually does not proceed along straight lines. Right panel: same situation, indicating identified nulls at (0.5316, 1.4514), (1.4755, 0.8766), and (2.0562, 1.8008), separatrices (gray), spine stream lines (green), and unit vectors of the actual flow field u (brown arrows).