assertthat is an extension to stopifnot() that makes it
    easy to declare the pre and post conditions that you code should
    satisfy, while also producing friendly error messages so that your
    users know what they've done wrong.
'assert-that.r' 'on-failure.r' 'assertions-file.r'
'assertions-scalar.R' 'assertions.r' 'base.r'
'base-comparison.r' 'base-is.r' 'base-logical.r' 'base-misc.r'
'utils.r' 'validate-that.R'