scala> ExampleParser.parse("UUU")
trying repUUU at scala.util.parsing.input.CharSequenceReader@638844a6
trying UUU at scala.util.parsing.input.CharSequenceReader@638844a6
UUU --> [1.4] parsed: P
trying UUU at scala.util.parsing.input.CharSequenceReader@76877f0f
UUU --> [1.4] failure: `UUU' expected but end of source found
UUU
^
repUUU --> [1.4] parsed: List(P)
trying optUAG at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UAG at scala.util.parsing.input.CharSequenceReader@76877f0f
UAG --> [1.4] failure: `UAG' expected but end of source found
UUU
^
optUAG --> [1.4] parsed: None
trying repUUU at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UUU at scala.util.parsing.input.CharSequenceReader@76877f0f
UUU --> [1.4] failure: `UUU' expected but end of source found
UUU
^
repUUU --> [1.4] parsed: List()
trying optUAG at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UAG at scala.util.parsing.input.CharSequenceReader@76877f0f
UAG --> [1.4] failure: `UAG' expected but end of source found
UUU
^
optUAG --> [1.4] parsed: None
trying repUUU at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UUU at scala.util.parsing.input.CharSequenceReader@76877f0f
UUU --> [1.4] failure: `UUU' expected but end of source found
UUU
^
repUUU --> [1.4] parsed: List()
trying optUAG at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UAG at scala.util.parsing.input.CharSequenceReader@76877f0f
UAG --> [1.4] failure: `UAG' expected but end of source found
UUU
^
optUAG --> [1.4] parsed: None
trying repUUU at scala.util.parsing.input.CharSequenceReader@76877f0f
trying UUU at scala.util.parsing.input.CharSequenceReader@76877f0f