call(name, ...) is.call(expr) as.call(expr)
name must be a quoted string which gives
the name of a function to be called).
is.call is used to determine whether expr is a call.
We don't differentiate between expressions and function calls.
So is.call is the same as is.expression.
It is not possible to coerce objects to mode call
(objects either are calls or they are not calls).
as.call returns its argument if it is a call and
otherwise terminated with an error message.