Your questions are really unclear. In the 2-parameters form `a[low:high]`, both values can be left out, defaulting to respectively 0 and len(a). In the 3-param

Your questions are really unclear. In the 2-parameters form `a[low:high]`, both values can be left out, defaulting to respectively 0 and len(a). In the 3-parameter form, only the leading value can be left out, defaulting to 0. I've no idea what (4) is asking about.