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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def from_spec(cls, spec):
 """ Load a Custodian instance where the jobs are specified from a structure and a spec dict. This allows simple custom job sequences to be constructed quickly via a YAML file. Args: spec (dict):
 A dict specifying job. A sample of the dict in YAML format for the usual MP workflow is given as follows ``` jobs: - jb: custodian.vasp.jobs.VaspJob params: final: False suffix: .relax1 - jb: custodian.vasp.jobs.VaspJob params: final: True suffix: .relax2 settings_override: {"file": "CONTCAR", "action": {"_file_copy": {"dest": "POSCAR"}} jobs_common_params: vasp_cmd: /opt/vasp handlers: - hdlr: custodian.vasp.handlers.VaspErrorHandler - hdlr: custodian.vasp.handlers.AliasingErrorHandler - hdlr: custodian.vasp.handlers.MeshSymmetryHandler validators: - vldr: custodian.vasp.validators.VasprunXMLValidator custodian_params: scratch_dir: /tmp ``` The `jobs` key is a list of jobs. Each job is specified via "job": <explicit path>, and all parameters are specified via `params` which is a dict. `common_params` specify a common set of parameters that are passed to all jobs, e.g., vasp_cmd. Returns: Custodian instance. """


        dec = MontyDecoder()

        def load_class(dotpath):
            modname, classname = dotpath.rsplit(".", 1)
            mod = __import__(modname, globals(), locals(), [classname], 0)
            return getattr(mod, classname)

        def process_params(d):
            decoded = {}
            for k, v in d.items():
                if k.startswith("$"):
                    if isinstance(v, list):
                        v = [os.path.expandvars(i) for i in v]
                    elif isinstance(v, dict):
                        v = {k2: os.path.expandvars(v2) for k2, v2 in v.items()}
                    else:
                        v = os.path.expandvars(v)
                decoded[k.strip("$")] = dec.process_decoded(v)
            return decoded

        jobs = []
        common_params = process_params(spec.get("jobs_common_params", {}))

        for d in spec["jobs"]:
            cls_ = load_class(d["jb"])
            params = process_params(d.get("params", {}))
            params.update(common_params)
            jobs.append(cls_(**params))

        handlers = []
        for d in spec.get("handlers", []):
            cls_ = load_class(d["hdlr"])
            params = process_params(d.get("params", {}))
            handlers.append(cls_(**params))

        validators = []
        for d in spec.get("validators", []):
            cls_ = load_class(d["vldr"])
            params = process_params(d.get("params", {}))
            validators.append(cls_(**params))

        custodian_params = process_params(spec.get("custodian_params", {}))

        return cls(jobs=jobs, handlers=handlers, validators=validators,
                   **custodian_params)