If trim is zero (the default), the arithmetic mean of the
  values in x is computed, as a numeric or complex vector of
  length one.  If x is not logical (coerced to numeric), numeric
  (including integer) or complex, NA_real_ is returned, with a warning.
If trim is non-zero, a symmetrically trimmed mean is computed
  with a fraction of trim observations deleted from each end
  before the mean is computed.