expression = ? Any valid Inko expression ?;
slicing    = expression, '[', expression, ']', [ '=', expression ];

Slice expressions are used to access to set the index of an object. These are written in the form receiver[index] and receiver[index] = value. These expressions are parsed into method calls, with receiver[index] translating to receiver.[](index), and receiver[index] = value translating to receiver.[]=(index, value).