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+"""Classes to track inputs/outputs of modeling protocols.
+
+   See also :class:`modelcif.protocol.Step`.
+"""
+
+
+class Data:
+    """Some part of the system that is input or output by part of
+       the modeling protocol, and/or stored in a file.
+
+       Usually a subclass is passed to :class:`modelcif.protocol.Step`
+       to describe the input or output, or to :class:`modelcif.associated.File`
+       to point to where the data are stored:
+
+        - A database of possible template sequences/structures to construct
+          or search (:class:`modelcif.ReferenceDatabase`)
+        - A template structure (:class:`modelcif.Template`)
+        - The sequence of the target (:class:`modelcif.Entity`)
+        - A target-template alignment (:mod:`modelcif.alignment`)
+        - Target structure coordinates (:class:`modelcif.model.Model`)
+
+       However, this class can also be used directly to describe other kinds
+       of input/output data.
+
+       :param str name: A short name for the data.
+       :param str details: A longer description of the data.
+    """
+    data_content_type = 'other'
+    data_other_details = None
+
+    def __init__(self, name, details=None):
+        self.name = name
+        self.data_other_details = details
+
+
+class DataGroup(list):
+    """A number of :class:`Data` objects that are grouped together.
+
+       This class can be used to group together multiple :class:`Data`
+       objects if a given modeling protocol step consumes or generates
+       multiple pieces of data. See :class:`modelcif.protocol.Step`. It behaves
+       like a regular Python list.
+    """
+    pass