Represents the "Function" entry of the type 6 shading dictionary
It is an optional entry defined as part of the PDF 1.0
It contains an array which contains FunctionElements.
If you read the specification you will see that an array of one
may be represented as a single FunctionElement
rather than an array. However we always present this property as an
array to simplify the interface. It is only if you add or remove
items in the array, that the underlying representation will be
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 84,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 83, page 245.