code: public Operator operator(String fldName) {
  		if (lhs.isFieldName() && lhs.asFieldName().equals(fldName))
  			return op;
  		if (rhs.isFieldName() && rhs.asFieldName().equals(fldName))
  			return op.complement();
  		return null;
  	}
docs: Determines if this term is of the form "F&lt;OP&gt;E" where F is the
  specified field, &lt;OP&gt; is an operator, and E is an expression. If
  so, the method returns &lt;OP&gt;. If not, the method returns null.
  
  @param fldName
  the name of the field
  @return either the operator or null