1. A method comprising: receiving, by an interface from a user, a definition of an instance of a concept, wherein the instance is specific to a particular field of knowledge of the user; in response to receiving the definition, generating, by a hardware processor, a user ontological model based on the definition, wherein the user ontological model is created for the user and the particular field of knowledge of the user, wherein a main ontological model provided to the user does not comprise ontological objects specific to the particular field of knowledge, wherein the user ontological model and the main ontological model are for use in natural language processing of natural language text, wherein the user ontological model comprises an instance structure that is based on the definition, and wherein the instance structure has an identifier; accessing a semantic-syntactic tree generated from a text provided by the user, wherein the semantic-syntactic tree comprises a plurality of connected nodes, and wherein at least one node among the plurality of connected nodes comprises a reference to the identifier of the instance structure; identifying the instance structure in the user ontological model based on the reference to the identifier in the semantic-syntactic tree; in response to identifying the instance structure, generating a first information object for the node in the semantic-syntactic tree based on the instance structure; and performing, by the hardware processor, natural language processing of the text based on the first information object.