You can remove rows from a matrix by applying a masking column.

To remove rows from a matrix according to some condition, build a mask and use the maskarray command. The mask must be an array of real with the same size as a column of the original matrix (nrows) filled with only 0's and 1's. Usually you make a mask by using evalformula with a boolean expression.
maskarray removes the rows which correspond to the 0.0 value in the mask.
set b to {0.0, 1.0, 10.0, 11.0, 20.0, 21.0} as array of real
set c to ArrayToMatrix(b, 2, 3)
set m to {0.0, 1.0, 1.0}
set d to maskarray c with m
display d
 Result:
10.000000 11.000000
20.000000 21.000000

Since {a, b, c} as matrix returns the matrix with columns a, b and c, the operations maskarray and as matrix commute.
(maskarray {a, b, c} with mk) as matrix
 is the same matrix as
maskarray ({a, b, c} as matrix) with mk
