A field selection: operand.field, or a map lookup written the same way.
test_only is true for a presence test, has(operand.field), whose type
is :bool. optional is true for operand.?field, whose type is
{:optional, t}.
Summary
Types
@type t() :: %CEL.Expr.Select{ field: String.t(), id: CEL.Checked.expr_id(), operand: CEL.Expr.t(), optional: boolean(), test_only: boolean(), type: CEL.Expr.type() }