[R] Extracting only the non-repeated rows of matrices

[dreamworx]

I have 2 matrices. One which contains a complete set of row combinations.
Another which contains a subset of the first. What I would like to extract
is the rows which are in the first but not repeated in the second. For
instance if the following were my two matrices,

A
       [,1] [,2] [,3] [,4]
  [1,]    1    2    3    4
  [2,]    1    2    3    5
  [3,]    1    2    3    6
  [4,]    1    2    3    7
  [5,]    1    2    3    8
  [6,]    1    2    3    9


B
       [,1] [,2] [,3] [,4]
  [1,]    1    2    3    4
  [2,]    1    2    3    5
  [3,]    1    2    3    6
  [4,]    1    2    3    9


I would like to extract the following,

       [,1] [,2] [,3] [,4]
  [1,]    1    2    3    7
  [2,]    1    2    3    8

I would like to know if there is a simple solution to this problem? If I
added the matrices and used the unique function, could I track which rows
have been deleted? Otherwise, I can only think of performing a loop with an
if statement.
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