diff --git a/docs/sophia_stdlib.md b/docs/sophia_stdlib.md
index 06357f4..73e3aa8 100644
--- a/docs/sophia_stdlib.md
+++ b/docs/sophia_stdlib.md
@@ -1635,9 +1635,15 @@ The described below functions take care of normalization of the fractions –
 they won't grow if it is unnecessary. Please note that the size of `frac` can be still
 very big while the value is actually very close to a natural number – the division of
 two extremely big prime numbers *will* be as big as both of them. To face this issue
-the [optimize](#optimize) function is provided. It will approximate the value of the
+the [optimize](#optimize** function is provided. It will approximate the value of the
 fraction to fit in the given error margin and to shrink its size as much as possible.
 
+**Important note:** `frac` must *not* be compared using standard `<`-like operators.
+The operator comparison is not possible to overload at this moment, nor the
+language provides checkers to prevent unintended usage of them. Therefore the typechecker
+**will** allow that and the results of such comparison will be unspecified.
+You should use [lt](#lt), [geq](#geq), [eq](#eq) etc instead.
+
 ### make_frac
 `Frac.make_frac(n : int, d : int) : frac`