public OptionalInt getStart() {
        return start == null ? OptionalInt.empty() : OptionalInt.of(start);
    }
Represents a slice expression, containing an optional zero-based
start offset, zero-based stop offset, and step.

@see <a href="https://jmespath.org/specification.html#slices">Slices</a>
smithy-jmespath/src/main/java/software/amazon/smithy/jmespath/ast/SliceExpression.java