correction
I thought I had already added in the missing `-1`. That it was still missing is worrisome.
MODIFY
README.md
README.md
@@ -14,7 +14,7 @@ these data types represent mathematical numbers (The JS number type's `NaN`,
its floating point data type, we'll always say "mathematical number" when
that's what we mean.
-This package is concerned the mathematical *natural numbers*, the non-negative
+This package is concerned with the mathematical *natural numbers*, the non-negative
integers. All of these can be safely represented as bigints, given enough
memory. Some of these can be represented as JS numbers, and a smaller set can
*safely* be represented as JS numbers, given a specific notion of safety.
@@ -30,7 +30,7 @@ A skippable detail about floating point:
that other integers will round to `2**53`. The JavaScript standard defines
the [*safe* JS numbers](https://tc39.es/ecma262/#sec-number.issafeinteger)
to be the JS numbers that represent mathematical integers and lie in the
- range `-(2**53-1)` to `2**53` . The JS safe natural numbers are the
+ range `-(2**53-1)` to `2**53-1` . The JS safe natural numbers are the
non-negative subset of that, between `0` and `2**53-1`. No other integers
coerce to any of these. If in JavaScript `a + b === c` and all three values
are JS safe integers, then this accurately represents the mathematical sum

correction

I thought I had already added in the missing `-1`. That it was still missing is worrisome.