sliceExpr method
Implementation
Parser<SliceNode> sliceExpr() =>
seq3(
ref0(singleExpression).optional(),
ref1(token, ':'),
seq2(
ref0(singleExpression).optional(),
seq2(ref1(token, ':'), ref0(singleExpression).optional()).optional(),
),
).map3((lower, _, rest) {
final upper = rest.$1;
final step = rest.$2?.$2;
return SliceNode(lower: lower, upper: upper, step: step);
});