fails:String#to_f stops if the underscore is not followed or preceded by a number
fails:String#to_f ignores leading and trailing whitespace
fails:String#to_f raises Encoding::CompatibilityError if String is in not ASCII-compatible encoding
fails:String#to_f allows String representation without a fractional part
fails:String#to_f stops if the underscore is not followed by a number
fails:String#to_f stops if +/- is not followed by a number
fails:String#to_f stops if the underscore appears at boundaries
