##' @title Random Sparse Matrix
##' @param nrow,
##' @param ncol number of rows and columns, i.e., the matrix dimension
##' @param nnz number of non-zero entries
##' @param rand.x random number generator for 'x' slot
##' @param ... optionally further arguments passed to sparseMatrix()
##' @return a sparseMatrix of dimension (nrow, ncol)
##' @author Martin Maechler