nrow_elements S3 method for blockmatrix objectnrow_elements S3 method for blockmatrix
  object
nrow_elements(M, zero_element = "0", ...)
  "nrow_elements" (M, zero_element = "0", ...)
  "nrow_elements" (M, zero_element = "0", ...)blockmatrix object"0"M.
  It is NA if the elements has different number of
  rows.