Systems and methods for content selection with first and second recommendation engines are disclosed herein. The system can include a memory include a content library database and a model database. The system can include a user device having a first network interface and a first I/O subsystem. The system can include one or more servers that can include a packet selection system and a presentation system. These one or more servers can: receive response data from the user device; provide received response data to a first recommendation engine; alert a second recommendation engine when a selected next node is a placeholder node; retrieve at least one statistical model relevant to selection of next node content; and select next node content based on an output of the at least one statistical model.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A system for content selection with first and second recommendation engines, the system comprising: memory comprising: a content library database comprising a plurality of nodes arranged in a content network, wherein the nodes in the content network are pairwise connected via a plurality of edges, wherein some of the nodes in the content network are associated with a data packet and a guard condition, and wherein some of the nodes in the content network are associated with a database of placeholder content; and a model database comprising a plurality of models relating to at least one of: a user skill level or a data packet difficulty level; a user device comprising: a first network interface configured to exchange data via a communication network; and a first I/O subsystem configured to convert electrical signals to user interpretable outputs via a user interface; and one or more servers comprising a packet selection system and a presentation system, wherein the one or more servers are configured to: receive response data from the user device; provide received response data to a first recommendation engine, wherein the first recommendation engine is configured to select a next node based on a current location in the content network; potential next nodes; past response data; and one or several guard conditions associated with the potential next nodes; alert a second recommendation engine when a selected next node comprises a placeholder node, wherein the placeholder node is associated with a database of placeholder content; retrieve at least one model relevant to selection of next node content; and select next node content based on an output of the at least one model. 2. The system of claim 1, wherein providing received response data to the first recommendation engine further comprises selecting a next node. 3. The system of claim 2, wherein selecting the next node comprises: identifying potential next nodes; and retrieving guard conditions, wherein each guard condition defines one or several prerequisites for entry into one of the potential next nodes. 4. The system of claim 2, wherein identifying potential next nodes comprises: identifying the user's location in the content network, wherein the user's location in the content network comprises an origin node; identifying edges extending from the origin node; and identifying non-prerequisite nodes connected to the origin node via the identified edges. 5. The system of claim 4, wherein selecting the next node further comprises: identifying the user associated with the user device; and retrieving the user history from the memory. 6. The system of claim 5, wherein selecting the next node further comprises application of the user history to the guard conditions of the potential next nodes. 7. The system of claim 6, wherein application of the user history to the guard conditions of the potential next nodes comprises: (a) selecting one of the potential next nodes; (b) identifying of a guard condition associated with the selected potential next node; (c) comparing the guard condition with the user history; and (d) associating a first value with the selected one of the potential next nodes when the comparison of the guard condition with the user history indicates that the guard condition is met. 8. The system of claim 7, wherein steps (a)-(d) are repeated for each of the potential next nodes. 9. The system of claim 8, wherein selecting next node content based on an output of the at least one model comprises: identifying the user associated with the user device; retrieving the user history; and identifying potential next node content. 10. The system of claim 9, wherein selecting next node content based on an output of the at least one model comprises: identifying one or several features of at least one of: the potential next node content or the user history; extracting the identified one or several features; and inputting some or all of the one or several features into the retrieved at least one model. 11. The system of claim 10, wherein selecting next node content based on an output of the at least one model comprises generating an output with the retrieved at least one model based on the input some or all of the one or several features. 12. The system of claim 11, wherein the one or more servers are further configured to provide the selected next node content to the user device. 13. The system of claim 12, wherein providing the selected next node content to the user device comprises: generating a plurality of electrical signals comprising the selected next node content; and sending the electrical signal to the user device. 14. The system of claim 1, wherein the first recommendation engine is in the presentation system and wherein the second recommendation engine is in the packet selection system. 15. A method for content selection with first and second recommendation engines, the method comprising: receiving response data from a user device at one or more servers comprising a packet selection system and a presentation system; automatically providing received response data to a first recommendation engine, wherein the first recommendation engine is configured to select a next node based on at least one of: a current location in the content network; potential next nodes; past response data; or one or several guard conditions associated with the potential next nodes; alerting a second recommendation engine when a selected next node comprises a placeholder node, wherein a placeholder node is associated with a database of placeholder content; retrieving at least one model relevant to selection of next node content from a model database comprising a plurality of models relating to at least one of: a user skill level or a data packet difficulty level; and selecting next node content based on an output of the at least one model. 16. The method of claim 15, further comprising providing the selected next node content to the user device. 17. The method of claim 16, wherein providing the selected next node content to the user device comprises: generating a plurality of electrical signals comprising the selected next node content; and sending the electrical signal to the user device. 18. The method of claim 17, wherein the first recommendation engine is in the presentation system and wherein the second recommendation engine is in the packet selection system. 19. The method of claim 17, wherein selecting next node content based on an output of the at least one model comprises: identifying the user associated with the user device; retrieving the user history; and identifying potential next node content. 20. The method of claim 19, wherein selecting next node content based on an output of the at least one model comprises: identifying one or several features of at least one of: the potential next node content or the user history; extracting the identified one or several features; and inputting some or all of the one or several features into the retrieved at least one model.