Moving minimums

x mmin y     mmin[x;y]

Where y is a non-symbol sortable list and x is a

• positive int atom, returns the x-item moving minimums of y, with nulls treated as the minimum value; the first x items of the result are the minimums of the terms so far, and thereafter the result is the moving minimum
• 0 or a negative int, returns y

q)3 mmin 0N -3 -2 1 -0W 0
0N 0N 0N -3 -0W -0W
q)3 mmin 0N -3 -2 1 0N -0W    / null is the minimum value
0N 0N 0N -3 0N 0N

mmin is a uniform function.