code: public String oppositeField(String fldName) {
  		if (lhs.isFieldName() && lhs.asFieldName().equals(fldName)
  				&& rhs.isFieldName())
  			return rhs.asFieldName();
  		if (rhs.isFieldName() && rhs.asFieldName().equals(fldName)
  				&& lhs.isFieldName())
  			return lhs.asFieldName();
  		return null;
  	}
docs: Determines if this term is of the form "F1&lt;OP&gt;F2" where F1 is the
  specified field, &lt;OP&gt; is an operator, and F2 is another field. If
  so, the method returns F2. If not, the method returns null.
  
  @param fldName
  the name of F1
  @return either the name of the other field, or null