Add missing file

src/traits.rs

@@ -0,0 +1,13 @@
+/// Estimate a statistic of a sequence of numbers ("population").
+pub trait Estimate {
+    /// Add an observation sampled from the population.
+    fn add(&mut self, x: f64);
+
+    /// Estimate the statistic of the population.
+    fn estimate(&self) -> f64;
+}
+
+/// Merge another sample into this one.
+pub trait Merge {
+    fn merge(&mut self, other: &Self);
+}