@classmethod
    def from_spec(cls, spec):
        """Creates a `ReadInstruction` instance out of a string spec.

        Args:
            spec (`str`):
                Split(s) + optional slice(s) to read + optional rounding
                if percents are used as the slicing unit. A slice can be specified,
                using absolute numbers (`int`) or percentages (`int`).

        Examples:

            ```
            test: test split.
            test + validation: test split + validation split.
            test[10:]: test split, minus its first 10 records.
            test[:10%]: first 10% records of test split.
            test[:20%](pct1_dropremainder): first 10% records, rounded with the pct1_dropremainder rounding.
            test[:-5%]+train[40%:60%]: first 95% of test + middle 20% of train.
            ```