"""
$(TYPEDEF)

An abstract type the subtypes of which are associated with functions method
overloaded for for new extensions. An instance of the `DefaultExt <:ExtensionType`
structure to the `Optimizer` in the `ext_type` field.
"""
abstract type ExtensionType end
struct DefaultExt <: ExtensionType end
MOIU.map_indices(::Function, x::ExtensionType) = x
MOIU.map_indices(::Function, x::DefaultExt) = x
