Added abbreviations.

ABV.md

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+# One-letter abbreviations
+
+These are highly context-dependent in their meaning, but in general:
+
+|Letter|Meaning(s)|
+|---|---|
+|`a`|Accumulator, generic first variable in sequence|
+|`b`|See `a` above|
+|`c`|Current value, see `a` above|
+|`d`|Difference, delta|
+|`e`|Expression, exception, error, event|
+|`f`|File, function|
+|`g`|Also a function|
+|`i`|Index, iterator, imaginary|
+|`j`|Also an index, also imaginary|
+|`l`|Length, list, long, last|
+|`n`|Number|
+|`o`|Object, out|
+|`p`|Pointer|
+|`r`|Return value, result|
+|`s`|Size|
+|`t`|Time, period|
+|`v`|Vector, variable|
+|`x`|First field of a vector, generic first variable in sequence, literally anything else|
+|`y`|Second field of a vector, see `x` above|
+|`z`|Third field of a vector, see `x` above, complex number|