[R] Get a copy of a matrix only for TRUE entries of a matching size boolean matrix?

[DynV Montrealer]

Is there a way to get a copy of a matrix only for TRUE entries of a
matching size boolean matrix? For *example*:
> mat_letters <- matrix(data=c('A', 'B', 'C', 'D'), ncol=2, byrow=TRUE)
> mat_letters
     [,1] [,2]
[1,] "A"  "B"
[2,] "C"  "D"
> mat_bools <- matrix(data=c(FALSE, TRUE, TRUE, FALSE), ncol=2, byrow=TRUE)
> mat_bools
      [,1]  [,2]
[1,] FALSE  TRUE
[2,]  TRUE FALSE
*Reminder:* The following is only an example ; the solution might look very
different.
some_command(mat_letters, mat_bools, false=empty)
     [,1] [,2]
[1,] ""  "B"
[2,] "C"  ""
some_command(mat_letters, mat_bools, false=na)
     [,1] [,2]
[1,] NA  "B"
[2,] "C"  NA

Thank you kindly

	[[alternative HTML version deleted]]