str_assert_not (PDF::Reader::Error)

    # File lib/pdf/reader/error.rb, line 38
        def self.str_assert_not (lvalue, rvalue, chars=nil)
          raise MalformedPDFError, "PDF malformed, expected string but found #{lvalue.class} instead" if chars and !lvalue.kind_of?(String)
          lvalue = lvalue[0,chars] if chars
          raise MalformedPDFError, "PDF malformed, expected '#{rvalue}' but found #{lvalue} instead"  if lvalue == rvalue
        end