This page tries to list all the symbols that are available from the standard library (algorithms library) in the namespace std::execution. The symbols are written as follows:
()
<>
P S U
par (since C++17) par_unseq (since C++17) parallel_policy (since C++17) parallel_unsequenced_policy (since C++17)
seq (since C++17) sequenced_policy (since C++17)
unseq (since C++20) unsequenced_policy (since C++20)