=. removes any rules defined for lhs.
Rules are removed only when their left-hand sides are identical to lhs, and the tests in Condition given on the right-hand side are also identical.
See the Mathematica book: Section 1.4.2, Section 2.4.3, Section A.5.4.
See also: Clear, TagUnset.
This assigns a value to the symbol a, uses that value in a calculation, unsets the definition, and tries the same calculation again.
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