If na.rm is FALSE an NA
  value in any of the arguments will cause
  a value of NA to be returned, otherwise
  NA values are ignored.
This is a generic function: methods can be defined for it
  directly or via the Summary group generic.
  For this to work properly, the arguments … should be
  unnamed, and dispatch is on the first argument.
Logical true values are regarded as one, false values as zero.
  For historical reasons, NULL is accepted and treated as if it
  were numeric(0).