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diff --git a/src/main/java/com/google/devtools/build/lib/query2/engine/SetExpression.java b/src/main/java/com/google/devtools/build/lib/query2/engine/SetExpression.java
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+++ b/src/main/java/com/google/devtools/build/lib/query2/engine/SetExpression.java
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+// Copyright 2014 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.query2.engine;
+
+import com.google.common.base.Joiner;
+
+import java.util.Collection;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Set;
+
+/**
+ * A set(word, ..., word) expression, which computes the union of zero or more
+ * target patterns separated by whitespace.  This is intended to support the
+ * use-case in which a set of labels written to a file by a previous query
+ * expression can be modified externally, then used as input to another query,
+ * like so:
+ *
+ * <pre>
+ * % blaze query 'somepath(foo, bar)' | grep ... | sed ... | awk ... >file
+ * % blaze query "kind(qux_library, set($(<file)))"
+ * </pre>
+ *
+ * <p>The grammar currently restricts the operands of set() to being zero or
+ * more words (target patterns), with no intervening punctuation.  In principle
+ * this could be extended to arbitrary expressions without grammatical
+ * ambiguity, but this seems excessively general for now.
+ *
+ * <pre>expr ::= SET '(' WORD * ')'</pre>
+ */
+class SetExpression extends QueryExpression {
+
+  private final List<TargetLiteral> words;
+
+  SetExpression(List<TargetLiteral> words) {
+    this.words = words;
+  }
+
+  @Override
+  public <T> Set<T> eval(QueryEnvironment<T> env) throws QueryException {
+    Set<T> result = new LinkedHashSet<>();
+    for (TargetLiteral expr : words) {
+      result.addAll(expr.eval(env));
+    }
+    return result;
+  }
+
+  @Override
+  public void collectTargetPatterns(Collection<String> literals) {
+    for (TargetLiteral expr : words) {
+      expr.collectTargetPatterns(literals);
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "set(" + Joiner.on(' ').join(words) + ")";
+  }
+}