Example implementation with pl-theorem-prover.
(define-plottable-dag pl-theorem-prover:node
:id #'pl-theorem-prover:node-id
:label (lambda (node)
(format nil "~A" (pl-theorem-prover:node-formula node)))
:children #'pl-theorem-prover:node-children
:color (lambda (node)
(if (pl-theorem-prover:node-truth-value node)
"#fff"
"#bbb")))