// Represents a key's relationship to a set of values.
  // Valid operators are In, NotIn, Exists, DoesNotExist. Gt, and Lt.
  optional string operator = 2;

  // An array of string values. If the operator is In or NotIn,
  // the values array must be non-empty. If the operator is Exists or DoesNotExist,
  // the values array must be empty. If the operator is Gt or Lt, the values

        assertEquals(out.getProperties().get("outputDirectory"), "${env.DOES_NOT_EXIST}");
    }

    @Test
    public void expressionThatEvaluatesToNullReturnsTheLiteralString() throws Exception {
        Map<String, String> modelProperties = new HashMap<>();
        modelProperties.put("outputDirectory", "${DOES_NOT_EXIST}");

        Model model = Model.newBuilder().properties(modelProperties).build();

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