boost::yap::then
// In header: <boost/yap/algorithm.hpp> template<typename Expr> decltype(auto) constexpr then(Expr && expr);
Returns the then-expression in an if_else expression.
Equivalent to get(expr, 1_c).
get(expr, 1_c)
then() is only valid if Expr is an expr_kind::if_else expression.
then()
expr_kind::if_else
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