Call by value-result vs. call by reference?
EDIT:
An addendum: if you want to "simulate" call by value-result in Pascal (or in another language that supports only call by reference); you can use this trick:

procedure p(var x, y : Integer); // simulates call by value-result
var
  xtmp, ytmp : Integer;

  procedure p_hidden(var x, y : Integer); // nested "hidden" procedure
  begin
  ... YOUR ORIGINAL CODE HERE ...
  end;

begin
  xtmp := x; ytmp := y; // use auxiliary variables
  p_hidden(xtmp,ytmp);  // call by ref the nested procedure
  x := xtmp; y := ytmp; // copy values before return 
end;


A simple method to "visualize" what is happening, is "inserting" the procedure in the point where it is called:

CALL BY REFERENCE:

a := 1; b := 2;   
p(a, b)
//// procedure p (var x, y : integer);
//// x is the same as a, y is the same as b
//// begin 
  a := a + 2 ;  // a = 3
  a := a * b ;  // a = 3 * 2 = 6
  a := a + 1    // a = a + 1 = 7
//// end; 
writeln (a)     // prints 7


CALL BY VALUE-RESULT:

a := 1; b := 2;  
p(a, b)
//// procedure p (valres x, y : integer); // by value result
//// x is the same as a, y is the same as b like in a by reference call
  xt := a; yt := b;  // xt and yt are two "new" variables
  //// begin 
  xt := xt + 2 ;  // xt = 3
  a := xt * yt ;  // a = 3 * 2 = 6
  xt := xt + 1    // xt = 3 + 1 = 4
//// end;  
//// the values of xt and yt are copied back to a and b (x and y) before return
  a := xt;       // a = 4
  b := yt;       // b = 2
writeln (a)      // prints 4