@dharr  Thanks for this. I've converted your comment into an answer. In essence, this is a ready-made help page for using prefix notation. In fact, this notation is very convenient and is widely used (in particular in programming), for example when the number of operands is large or a priori unknown.

@sand15  Of course your version is simpler. Thank you.

@nm  Yes, your approach is more consistent with the wishes of the OP. In this regard, we can introduce the concept of a sublist of a given list, that is, will the list  N  be a sublist of the list  L ?

@randomuser24  Specifications of a function  f(x):= ...  or   f:=x-> ...  are equivalent in recent versions of Maple.
Examples:

 f(x):=2*x^2-4*x:
plot([f(x), 2*f(x), 4*f(x), -f(x), -2*f(x), -4*f(x)], x=-1..3, color=[red,blue,green,cyan,khaki,brown]);

f:=x->2*x^2-4*x:
plot([f(x), 2*f(x), 4*f(x), -f(x), -2*f(x), -4*f(x)], x=-1..3, color=[red,blue,green,cyan,khaki,brown]);

Don't forget to put multiplication signs where necessary; they are often missed in your text.

@C_R Yes, this is possible in Maple 2018 as well. Thank you very much.

@acer  and @ Axel Vogt  Thank you very much! 

@JAMET  Look carefully at my answer about this (multiplying a matrix by a vector).

@JAMET 

restart;
with(linalg):
xA := 4;
yA := 10;
xB := 0;
yB := 0;
xC := 13;
yC := 0;
Mat := matrix(3, 3, [xA, xB, xC, yA, yB, yC, 1, 1, 1]);
eta:=(x,y,z)->evalm(Mat&*[x,y,z]);
eta(1,2,3);  

@mmcdara  
1. Thank you for your insightful analysis of the examples discussed. I am aware of these shortcomings in the program and will try to fix them as soon as I have enough free time.

2. You write that the program is too complicated. But how will you solve a similar problem if the number  n  is large enough. The first step  combinat:-partition(100)  or  combinat:-partition(100, 9)  will take too long, even if Maple has enough memory for this. My program is quite easy to cope with the problem  Partition(100, 1..100, {2,3,6,9});

@yangtheary  I don't understand what kind of help you need. Is there anything you don't understand about the code?

@vv   c<>5

@Maqroll  Consider 2 ways to define the function: h:=x->expr  and  h:=unapply(expr, x) , where  expr  is an expression. If the expression  expr  does not require any evaluation, then these 2 ways are equivalent, for example   h:=x->x^2+1  and  h:=unapply(x^2+1, x) . 
If  expr  requires calculation, for example, it is given as an integral as in your example or in some other way, then at the moment of the definition of an arrow-function it is not calculated, but will be calculated only when you call this function with some argument value. In the case of unapply-function, the expression  expr  is evaluated immediately when this function is specified. Compare

f := x->a*x/(4*x^2+b):
h:=x->int(f(t),  t=0..x, continuous);
h(x);
h(1);
 	
h:=unapply(int(f(t),  t=0..x, continuous), x);
h(1);

To do this, you can use piecewise command in the inert form. To make the brace blue instead of grey, we use InertForm:-Display command.

An example:

Sys:={x+y-z = 3, x-y-z = 5, -x-y-z = 7}:
%piecewise(``, Sys[1], ``, Sys[2], ``, Sys[3]);
InertForm:-Display(%, inert=false);

@Prakash J  To find the values of these parameters, as well as the value of the alpha parameter, some additional conditions are needed.

@C_R  You can easily plot a cube with this method:

plots:-implicitplot3d(max(-x,x-1,-y,y-1,-z,z-1), x=-0.5..1.5, y=-0.5..1.5, z=-0.5..1.5, style=surface, axes=normal, grid=[100,100,100]);