```I've worked it out in my mind why ++ isn't valid.  Maybe my thoughts will
help others.  And maybe someone can straighten me out if I'm wrong!

Following is an annotated irb session:

irb(main):001:0> a="abc"
"abc"
irb(main):002:0> a.id   # The "abc" string has a particular ID
22443468
irb(main):003:0> a.succ!# succ!() is only similar to ++ for numbers
"abd"                   # but is actually a well defined method
irb(main):004:0> a.id   # for introducing an effect on the string.
22443468                # Interestingly, we KEEP the same instance
# and only the contents had to change!
irb(main):005:0> a += "efg"
"abdefg"
irb(main):006:0> a
"abdefg"
irb(main):007:0> a.id   # With the above operation, we had to get a new
22410024                # instance since we did an assignment.
#
irb(main):008:0> b=123  # Now consider Integers:
123                     #
irb(main):009:0> b.id   # Now we have a particular Integer object.
247                     # Indeed, all 123 integers will have this ID
irb(main):010:0> b++    #
irb(main):011:0*        # As you can see, this op is invalid.  To be
irb(main):012:0*        # like succ!, it would require keeping the
irb(main):013:0* d      # SAME object but altering that objects CONTENTS
# (said contents being the immutable number 123).
# Compare the annotations for a.succ! and b++
# to convince yourself.
Error: ...              # But Integer values are immutable!
# Otherwise, the number 3 could become 4 and
# you'd end up with a silly Monty Python skit.
# ["1, 2, 4" "NO, 3, Sir!" "OH, 3!" BOOM!]
#
irb(main):014:0> b += 5 # Now this operation works because the language
128                     # 'knows' how to find the item in the set of
irb(main):015:0> b.id   # Integer values that is '5' beyond '123' and
257                     # Ruby then returns then assigns the reference
# of this newly found '128' object to the
# variable.  NOTE: assignment of a new instance
# has happened just as in the String += op.

The important part of the explanation is the b++ annotation.

I hope I have it right.  And if I do have it right, I hope it's the kind of
explanation that makes sense to someone else and can enlighten.

Good day.

+------------------------------------------------+
| DREW MILLS            | 10101 Linn Station Rd. |
|                       | Suite 800              |
| tamills / ups.com       | Louisville, KY  40223  |
| Technical Specialist  | <v> 502-394-7785       |
| United Parcel Service | <f> 502-394-7812       |
+------------------------------------------------+

```