/// <summary>
/// Coerce the specified expression to an S4 object.
/// </summary>
/// <param name="expression">The expression.</param>
/// <returns>The DataFrame. Returns <c>null</c> if the specified expression is not vector.</returns>
public static S4Object AsS4(this SymbolicExpression expression)
{
if (!expression.IsS4())
{
return(null);
}
return(new S4Object(expression.Engine, expression.DangerousGetHandle()));
}
