An R function taking a numeric first argument and
returning a numeric vector of the same length.
Returning a non-finite element will generate an error.
lower
Numeric, the lower limit of integration. Can be infinite.
upper
Numeric, the upper limit of integration. Can be infinite.
...
Addtional arguments for f.
show.warning
logical, display warning messages.
verbose
logical, display timing information, for debugging purpose.