http://lucene.apache.org/lucene-parent/lucene-expressions: Dynamically computed values to sort/facet/search on based on a pluggable grammar. (The Apache Software Foundation) 
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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,
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package org.apache.lucene.expressions;

import org.apache.lucene.search.DoubleValuesSource;


Binds variable names in expressions to actual data.


These are typically DocValues fields/FieldCache, the document's 
relevance score, or other ValueSources.

@lucene.experimental
/**
 * Binds variable names in expressions to actual data.
 * <p>
 * These are typically DocValues fields/FieldCache, the document's 
 * relevance score, or other ValueSources.
 * 
 * @lucene.experimental
 */
public abstract class Bindings {

  
Sole constructor. (For invocation by subclass 
 constructors, typically implicit.) 
/** Sole constructor. (For invocation by subclass 
   *  constructors, typically implicit.) */
  protected Bindings() {}

  
Returns a DoubleValuesSource bound to the variable name

/**
   * Returns a DoubleValuesSource bound to the variable name
   */
  public abstract DoubleValuesSource getDoubleValuesSource(String name);

}