How to restrict a search to specific combinatyions of terms.

How can I restrict a search in Devonagent to specific combinations of terms. For example if I search for x AND/NEAR y OR z, I get pages with all three terms. But I only want x if y or z are present. If there are no such pages I should get a nil return.
This is probably very basic stuff but I am new to all this.
Many thanks

Harry NEAR/4 (Tom OR Jim)