What is meant by "graphics primitive" in the sage documentation?

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In a sense, a graphics primitive is a component of a graphics object.

One way of creating a graphics object is to add several pre-existing ones.

In that case, you get a graphics object consisting of several graphics primitive.

Here is an example.

sage: a = plot(lambda x: 1)
sage: b = plot(lambda x: x)
sage: c = plot(lambda x: x^2)
sage: for g in (a, b, c, a + b, a + b + c):
....:     print g
....:
Graphics object consisting of 1 graphics primitive
Graphics object consisting of 1 graphics primitive
Graphics object consisting of 1 graphics primitive
Graphics object consisting of 2 graphics primitives
Graphics object consisting of 3 graphics primitives