@Carl Love Sorry, the parabola is not showing up in that plot! I don't know why. It'll take me a little while to figure it out.

@goli You simply need to terminate the display command with a semicolon rather than a colon. The purpose of the colon is to suppress the output, which in this case is the plot itself.

display(a1, a2);

@goli Sorry, but I don't know the first thing about copy-and-pasting. I do all my work programmatically. Plots can be merged using the command plots:-display. There's probably a way to do it with copy-and-paste; I just know nothing about it except that it leads to corruption, moral decay, and irreproducible results; it's antithetical to the scientific method.

@vv Yes, that's what I meant. Note that all code in seq, add, and mul statements is invoked for every index value.

Alias allows an existing rtable to be indexed in a different way, without copying it. So it's not quite correct to say that it creates a 2x2 Matrix. Rather, it allows the 1-D Array created inside the Iterator to be indexed as a Matrix. 

@vv It's not quite the same thing, although it may ultimately produce the same output. The point of my form is to avoid repeated calls to Alias.

@Stretto Maple's normal order of evaluation---used in the vast majority of cases---is from the inside to the outside. This can be changed by procedures that declare parameters as uneval. This should be used rarely, and with caution. EV must be such a procedure. I am not familiar with it. What is it?

This works fine:

Color(ithprime(5));

I get a green 11.

Your Wirth syntax diagram for the second form (the in form) is incorrect: The for clause is actually optional. Here's how that can be used in practice:

M:= ArrayTools:-Alias(output((J:= Iterator:-Combination(6,4))), 0, [2,2]):
(in J do M+~1 od); 
 

Please post an example of that, and please indicate your Maple version. You can post a worksheet using the green uparrow on the MaplePrimes editor toolbar.

@Carl Love My experience with Feedly so far is that

1. There's a lag of about an hour before messages appear on it;
2. I can't find any option for it to send notifications, i.e., to make my phone beep. 

It seems that you intended to attach a worksheet, but there's none attached. Please try again.

@Scot Gould Thanks, I will try Feedly. At this point, I know so little about this that I couldn't tell you if it's what I want. I realized that I'd get some spam notifications also, but that's okay since I'm one of the people who deletes them.

@Joe Riel An operator with a type declaration requires parentheses around the parameter declaration. This affects your second example.

If you have any questions about the above worksheet, I'd be happy to answer them.

If the assignment had just asked for a table of the corrected values, that could've been done much more easily with a stock Maple command: 

dsolve(
   ..., numeric, method= classical[heunform], stepsize= h, output= Array(h*~[$0..n])
);

But to also get the predicted values requires the work to be done at a low-level and in a loop. I edited my Answer to also give these stock Maple corrected values for comparison purposes.

@Stretto You can override that behavior for 0 or 1 arguments while keeping the original behavior for more arguments like this:

`%+`:= ()-> `if`(nargs=0, 0, `if`(nargs=1, args, 'procname'(args))):

I wrote a substantial Answer to your Question. It offends me that you haven't responded.