terminal_state.jl

"""
    TerminalState

A type with no fields whose singleton instance `terminalstate` is used to represent a terminal state with no additional information.

This type has the appropriate promotion logic implemented to function like `Missing` when added to arrays, etc.

Note that terminal states NEED NOT be of type `TerminalState`. You can define any state to be terminal by implementing the appropriate `isterminal` method. Solvers and simulators SHOULD NOT check for this type, but should instead check using `isterminal`. 
"""
struct TerminalState end

"""
    terminalstate

The singleton instance of type `TerminalState` representing a terminal state.
"""
const terminalstate = TerminalState()

isterminal(m::Union{MDP,POMDP}, ts::TerminalState) = true
Base.promote_rule(::Type{TerminalState}, T::Type) = Union{TerminalState, T}