|kind = None|
This class is a node of a relational expression. Leaves are relations and internal nodes are operations. The kind property says if the node is a binary operator, unary operator or relation. Since relations are leaves, a relation node will have no attribute for children. If the node is a binary operator, it will have left and right properties. If the node is a unary operator, it will have a child, pointing to the child node and a prop containing the string with the props of the operation. This class is used to convert an expression into python code.