added ex061 coercions

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diff --git a/exercises/061_coercions.zig b/exercises/061_coercions.zig
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+//
+// It'll only take us a moment to learn the Zig type coercion
+// rules because they're quite logical.
+//
+// 1. Types can always be made _more_ restrictive.
+//
+//    var foo: u8 = 5;
+//    var p1: *u8 = &foo;
+//    var p2: *const u8 = p1; // mutable to immutable
+//
+// 2. Numeric types can coerce to _larger_ types.
+//
+//    var n1: u8 = 5;
+//    var n2: u16 = n8; // integer "widening"
+//
+//    var n3: f16 = 42.0;
+//    var n4: f32 = n3; // float "widening"
+//
+// 3. Single-item pointers to arrays coerce to slices and
+//    many-item pointers.
+//
+//    const arr: [3]u8 = [3]u8{5, 6, 7};
+//    const s: []const u8 = &arr;  // to slice
+//    const p: [*]const u8 = &arr; // to many-item pointer
+//    
+// 4. Single-item mutable pointers can coerce to single-item
+//    pointers pointing to an array of length 1. (Interesting!)
+//
+//    var five: u8 = 5;
+//    var a_five: *[1]u8 = &five;
+//
+// 5. Payload types and null coerce to optionals.
+//
+//    var num: u8 = 5;
+//    var maybe_num: ?u8 = num; // payload type
+//    maybe_num = null;         // null
+//
+// 6. Payload types and errors coerce to error unions.
+//
+//    const MyError = error{Argh};
+//    var char: u8 = 'x';
+//    var char_or_die: MyError!u8 = char; // payload type
+//    char_or_die = MyError.Argh;         // error
+//
+// 7. 'undefined' coerces to any type (or it wouldn't work!)
+//
+// 8. Compile-time numbers coerce to compatible types.
+//
+//    Just about every single exercise program has had an example
+//    of this, but a full and proper explanation is coming your
+//    way soon in the third-eye-opening subject of comptime.
+//
+// 9. Tagged unions coerce to the current tagged enum.
+//
+// 10. Enums coerce to a tagged union when that tagged field is a
+//     a zero-length type that has only one value (like void).
+//
+// 11. Zero-bit types (like void) can be coerced into single-item
+//     pointers.
+//
+// The last three are fairly esoteric, but you're more than
+// welcome to read more about them in the official Zig language
+// documentation and write your own experiments.
+
+const print = @import("std").debug.print;
+
+pub fn main() void {
+    var letter: u8 = 'A';
+
+    const my_letter:   ???   = &letter;
+    //               ^^^^^^^
+    //           Your type here.
+    // Must coerce from &letter (which is a *u8).
+    // Hint: Use coercion Rules 4 and 5.
+
+    // When it's right, this will work:
+    print("Letter: {u}\n", .{my_letter.?.*[0]});
+}