This helper allows a calling application to require a given expression be
some sort of list constructor (tuple constructor in native syntax, or
array in JSON) and peel off that outer level of list to obtain a slice
of the Expression objects inside.
This is useful in rare cases where the calling application needs to
extract the expressions within the list without evaluating the entire list
expression first. For example, the expressions that result from this
function might be passed into AbsTraversalForExpr in situations where the
caller requires a static list of static traversals that will never
actually be evaluated.
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