Set-diagram references

• Euler diagram definition (Encyclopaedia of Mathematics) — prior art distinguishing diagrams that show only existing set intersections.
• Venn’s original set diagram paper — historical prior art for systematic set intersections.
• W3C SVG accessibility guidance — relevant when a set diagram is delivered as SVG and needs meaningful text alternatives.

No supported diagram.zip renderer currently guarantees a valid Venn or Euler layout from arbitrary set membership data. Treat overlap geometry as illustrative unless an integration adds validation.