Most efficient way to find related terms

I'm wondering what the most efficient approach is to finding all terms which are related to a specific term X. By related I mean terms which appear in a specific document field that also contains the target term X. e.g. Document has a keyword field, field1 that can contain multiple keywords

Translating Lucene Query Syntax to Traditional Boolean Syntax

We have an application that performs searches against a Lucene based index and also against a Windows Desktop Search based index. For simple queries we'd like to offer our users a consistent interface that allows them to build basic Lucene style queries using the 'MUST HAVE' (+), 'MUST NOT HAVE'