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diff --git a/exercises/024_errors4.zig b/exercises/024_errors4.zig
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+//
+// Using `catch` to replace an error with a default value is a bit
+// of a blunt instrument since it doesn't matter what the error is.
+//
+// Catch lets us capture the error value and perform additional
+// actions with this form:
+//
+//     canFail() catch |err| {
+//         if (err == FishError.TunaMalfunction) {
+//             ...
+//         }
+//     };
+//
+const std = @import("std");
+
+const MyNumberError = error{
+    TooSmall,
+    TooBig,
+};
+
+pub fn main() void {
+    // The "catch 0" below is just our way of dealing with the fact
+    // that makeJustRight() returns a error union (for now).
+    var a: u32 = makeJustRight(44) catch 0;
+    var b: u32 = makeJustRight(14) catch 0;
+    var c: u32 = makeJustRight(4) catch 0;
+
+    std.debug.print("a={}, b={}, c={}", .{ a, b, c });
+}
+
+// In this silly example we've split the responsibility of making
+// a number just right into four (!) functions:
+//
+//     makeJustRight()   Calls fixTooBig(), cannot fix any errors.
+//     fixTooBig()       Calls fixTooSmall(), fixes TooBig errors.
+//     fixTooSmall()     Calls detectProblems(), fixes TooSmall errors.
+//     detectProblems()  Returns the number or an error.
+//
+fn makeJustRight(n: u32) MyNumberError!u32 {
+    return fixTooBig(n) catch |err| {
+        return err;
+    };
+}
+
+fn fixTooBig(n: u32) MyNumberError!u32 {
+    return fixTooSmall(n) catch |err| {
+        if (err == MyNumberError.TooBig) {
+            return 20;
+        }
+
+        return err;
+    };
+}
+
+fn fixTooSmall(n: u32) MyNumberError!u32 {
+    // Oh dear, this is missing a lot! But don't worry, it's nearly
+    // identical to fixTooBig() above.
+    //
+    // If we get a TooSmall error, we should return 10.
+    // If we get any other error, we should return that error.
+    // Otherwise, we return the u32 number.
+    return detectProblems(n) ??? 
+}
+
+fn detectProblems(n: u32) MyNumberError!u32 {
+    if (n < 10) return MyNumberError.TooSmall;
+    if (n > 20) return MyNumberError.TooBig;
+    return n;
+}