**`ast.Set(elts)`** is a class defined in the `ast` module that is used to express Python tuples in the form of an #key# Abstract Syntax Tree: tree representation of source code #key#. When the `parse()` method of `ast` is called on a Python source code that contains a set, the `ast.Set` class is invoked and expresses the set to a node in an `ast` tree data structure. The `ast.Set` class represents the *Tuple* node type in the `ast` tree. In the class parameter, `elts` contains the list of nodes representing the elements in the set. 

# Example

import ast
from pprint import pprint

class SetVisitor(ast.NodeVisitor):

    def visit_Set(self, node):
        print('Node type: Set\nFields: ', node._fields)
        ast.NodeVisitor.generic_visit(self, node)


    def visit_Constant(self, node):
        print('Node type: Constant\nFields: ', node._fields)
        ast.NodeVisitor.generic_visit(self, node)


visitor = SetVisitor()
tree = ast.parse('{2,4,6}', mode='eval')
pprint(ast.dump(tree))
visitor.visit(tree)

# Explanation
* We define a `SetVisitor` class that extends from the parent class `ast.NodeVisitor`. We override the predefined `visit_Constant` and `visit_Set` methods in the parent class, which receive the `Constant` and `Set` nodes, respectively. In the method, we print the type and the fields inside the node and call the `generic_visit()` method, which invokes the propagation of the visit on the children nodes of the input node.
* We initialize a `visitor` object of the class `SetVisitor`.
* We define a Python set `{2,4,6}` and send it to the `ast.parse()` method with `mode='eval'`, which returns the result of the expression after evaluation and stores it in `tree`. 
* The `ast.dump()` method returns a formatted string of the tree structure in `tree`.
* The `visit` method available to the `visitor` objects visits all the nodes in the tree structure.

What is ast.Set(elts) in Python?

`ast.Set(elts)` represents Python sets in an Abstract Syntax Tree. It stores elements in the `elts` field.