CAS command question

[compsystems]

Hello Mr. Parisse

c:=a-b; [enter] Internally is a+neg(b) // ok

It's new to my, extracting parts of the expression using []
c[1]; -> +
c[2]; -> a
c[3]; -> b

Please explain, the correspondence of the position or numeral to extract the parts. thanks

for DrD
If you enter the expression in quotation marks, the LEFT or RIGHT command works as it should, the problem is that the user should always enter the parameters between ' ' =(

My question for Parisse: how is it done within a program to convert the input inside a quotation mark?

I try the following, but it does not work

PHP Code:

#cas
  helpquote( id0 ):=
  begin
    local id1;
    id1 := quote(id0); // -> 'id0' =(
    print; print(id1);

    print( [ left(id1), right(id1) ] ); wait;

    return ( [ left(id1), right(id1) ] );
  end;
  // helpquote( (x+y)^(1+1)-3 ) -> [(x+y)^(1+1),-3] //  This would be the expected output.
  // helpquote( 4/2 ) -> [4 2] // This would be the expected output.
#end 


so we would need a new CAS command called RECALL

PHP Code:

id1 := quote( RECALL (id0) ); 


Another solution would be to interpret as a QUOTE EXPRESSION, if the arguments are within quotes,

PHP Code:

helpquote( 'id0' ):=
  begin ...