[R] Judging if a matrix contains any NA

[Steven Yen]

How do I judge if a matrix contain any NA or otherwise non-missing, 
non-numerical?
In the following, I would like to deliver ONE logical of TRUE or FALSE, 
rather than a 4 x 4 matrix containing TRUE or FALSE. Thank you.

 > a<-matrix(1:16,nrow=4)
 > diag(a)<-NA
 > a
      [,1] [,2] [,3] [,4]
[1,]   NA    5    9   13
[2,]    2   NA   10   14
[3,]    3    7   NA   15
[4,]    4    8   12   NA
 > is.na(a)
       [,1]  [,2]  [,3]  [,4]
[1,]  TRUE FALSE FALSE FALSE
[2,] FALSE  TRUE FALSE FALSE
[3,] FALSE FALSE  TRUE FALSE
[4,] FALSE FALSE FALSE  TRUE