diff --git a/src/sometimes.zig b/src/sometimes.zig
new file mode 100644
index 0000000..dabca6c
--- /dev/null
+++ b/src/sometimes.zig
@@ -0,0 +1,103 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const SourceLocation = std.builtin.SourceLocation;
+
+// Registry of "sometimes" assertions.
+//
+// A sometimes-assertion records that a given condition was observed to be true
+// at least once during a simulation. Unlike a regular assertion (which must
+// always hold), a sometimes-assertion documents a branch we *expect* to be
+// exercised eventually. In a single run it may legitimately never fire; its
+// signal is only meaningful as coverage across many seeds.
+//
+// Each call site is identified by its source location (@src()), so the same
+// physical assertSometimes() call is one entry regardless of how many times
+// it is evaluated. An optional human-readable label is shown in the report.
+//
+// The registry only knows about assertions that were *evaluated* at least once.
+// A call site that is reached with the condition true at least once is reported
+// as taken; one that is reached but whose condition is always false is reported
+// as not-taken. A call site that is never reached at all cannot appear.
+
+pub const Sometimes = struct {
+    const Entry = struct {
+        file: []const u8,
+        fn_name: []const u8,
+        line: u32,
+        column: u32,
+        label: ?[]const u8,
+        eval_count: u64,
+        true_count: u64,
+
+        fn matches(self: Entry, src: SourceLocation) bool {
+            return self.line == src.line and
+                self.column == src.column and
+                std.mem.eql(u8, self.file, src.file);
+        }
+    };
+
+    gpa: Allocator,
+    entries: std.ArrayList(Entry),
+    reported: bool,
+
+    pub fn init(self: *Sometimes, gpa: Allocator) void {
+        self.gpa = gpa;
+        self.entries = .empty;
+        self.reported = false;
+    }
+
+    pub fn deinit(self: *Sometimes) void {
+        self.entries.deinit(self.gpa);
+    }
+
+    // Records one evaluation of a sometimes-assertion. The strings carried by
+    // @src() (and any string-literal label) live for the whole program, so we
+    // can store the slices directly without duplicating them.
+    pub fn record(self: *Sometimes, cond: bool, src: SourceLocation, label: ?[]const u8) void {
+        for (self.entries.items) |*entry| {
+            if (entry.matches(src)) {
+                entry.eval_count += 1;
+                if (cond) entry.true_count += 1;
+                if (label != null and entry.label == null) entry.label = label;
+                return;
+            }
+        }
+
+        // First time this call site is seen. If we can't grow the registry we
+        // simply drop the entry: a sometimes-assertion must never affect the
+        // behaviour of the program under test.
+        self.entries.append(self.gpa, .{
+            .file = src.file,
+            .fn_name = src.fn_name,
+            .line = src.line,
+            .column = src.column,
+            .label = label,
+            .eval_count = 1,
+            .true_count = if (cond) 1 else 0,
+        }) catch return;
+    }
+
+    pub fn report(self: *Sometimes) void {
+        self.reported = true;
+
+        if (self.entries.items.len == 0)
+            return;
+
+        var taken: usize = 0;
+        for (self.entries.items) |entry| {
+            if (entry.true_count > 0) taken += 1;
+        }
+
+        std.debug.print("\n=== Sometimes assertions: {d}/{d} taken ===\n", .{ taken, self.entries.items.len });
+        for (self.entries.items) |entry| {
+            const mark = if (entry.true_count > 0) "\u{2713}" else "\u{2717}";
+            std.debug.print("  [{s}] {s}:{d}:{d} ({s})", .{
+                mark, entry.file, entry.line, entry.column, entry.fn_name,
+            });
+            if (entry.label) |label|
+                std.debug.print(" \"{s}\"", .{label});
+            std.debug.print(" \u{2014} {d}/{d} true\n", .{ entry.true_count, entry.eval_count });
+        }
+        std.debug.print("\n", .{});
+    }
+};