question: private String optNodeDigest ( ParseTreeNode node ) { StringBuilder sb = new StringBuilder ( ) ; sb . append ( '(' ) ; for ( ParseTreeNode child : node . children ( ) ) { nodeDigest ( child , sb ) ; if ( sb . length ( ) > longestKeyLength ) { return null ; } } nodeTail ( node , sb ) ; if ( sb . length ( ) > longestKeyLength ) { return null ; } return sb . toString ( ) ; }
target:  Returns the digest of node, or null if the digest would be longer than the longest key in the knowledge base, and thus unable to match any key in the knowledge base.