Function as.brob() is the user's workhorse: use this to coerce
numeric vectors to brobs.Function is.brob() tests for its arguments being of class
brob.
Function brob() takes argument x and returns a brob
formally equal to $exp(x)$; set argument positive to
FALSE to return $-exp(x)$. Thus calling function
exp(x) simply returns brob(x). This function is not
really intended for the end user: it is confusing and includes no
argument checking. In general numerical work, use function
as.brob() instead, although be aware that if you really really
want $e^1e7$, you should use brob(1e7);
this would be an exact representation.