22. The computer program product of claim 17 where the computer-readable program code is configured to perform identifying at least one child node within the grammar that is directly traversable from a parent node within the grammar, where the parent node is a non-terminal element within the grammar corresponding to a non-terminal display object in the displayed expression, and where the child node is one of a terminal element within the grammar and a non-terminal element within the grammar.
22. The computer program product of claim 17 where the computer-readable program code is configured to perform identifying at least one child node within the grammar that is directly traversable from a parent node within the grammar, where the parent node is a non-terminal element within the grammar corresponding to a non-terminal display object in the displayed expression, and where the child node is one of a terminal element within the grammar and a non-terminal element within the grammar. 23. The computer program product of claim 22 where the computer-readable program code is configured to perform identifying the child node responsive to receiving selection of the non-terminal display object within the displayed expression.