fails:String#to_r returns a Rational object
fails:String#to_r returns (0/1) for the empty String
fails:String#to_r returns (n/1) for a String starting with a decimal _n_
fails:String#to_r ignores trailing characters
fails:String#to_r ignores leading spaces
fails:String#to_r does not ignore arbitrary, non-numeric leading characters
fails:String#to_r treats leading hypens as minus signs
fails:String#to_r does not treat a leading period without a numeric prefix as a decimal point
fails:String#to_r understands decimal points
fails:String#to_r ignores underscores between numbers
fails:String#to_r understands a forward slash as separating the numerator from the denominator
fails:String#to_r returns (0/1) for Strings it can't parse
