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or a descriptor of a corresponding operator to apply at that point.

The pointwise indicators or descriptors can be any relevant signs
or symbolic expressions:  address pointers, code numbers, names,
or quoted phrases.

A &quot;vector field&quot; is a construct that attaches to each point of
phase space a single tangent vector or differential operator.

A &quot;control system&quot; may be viewed as a ready generalization of a vector field,
in which whole sets of tangent vectors or differential operators 