Extending suggestion for stopifnot

character string at the end of an existing function, stopifnot().  (2)
I think "estrings" (that behave like characters but are interpolated
before printout) are useful in the same way perl interpolated strings
are useful.

The gsubfn package has string interpolation somewhat like perl.
Preface a function call with fn$ and then back ticks and $ are
interpolated.

library(gsubfn)
fn$identity("pi is $pi")

library(sqldf)
fn$sqldf("select * from BOD where Time > $pi")

fn$stop("class is `class(pi)`")