Modifying state of shopping cart is unsafe since produces side-effect:
"Add to Cart" an HTML link:
<a href=".../AddToShoppingCart?docNumber=6,678,889...">...
I cannot link to shopping cart from this slide; a search engine or pre-fetching agent might increment counter (cf. SVG 1.2, section 11.8.
In HTML, use "POST" form for unsafe operations.
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