Expressions are used to define filter logic for passing to a Dataset
Scanner.
Expression$scalar(x) constructs an Expression which always evaluates to
the provided scalar (length-1) R value.
Expression$field_ref(name) is used to construct an Expression which
evaluates to the named column in the Dataset against which it is evaluated.
Expression$create(function_name, ..., options) builds a function-call
Expression containing one or more Expressions.