On Fri, 01 Jun 2018 20:29:41 +0900
Jeong Dal <hak...@me.com> wrote:

> After inserting a line “save x, y;”, it works.

This is also defined as "clearxy ;"

For those learning MetaPost, simplifying, z is defined as "(x,y)" so
one can write
        z = (a,b) ;
This is an EQUATION in MetaPost
        (x,y) = (a,b) ;
not an assignment (as in most other languages), actually a pair of
        x = a ; y = b ;
When x and y are unknown, this can be "solved". When x and y contain
values, it leads to "inconsistent equations".

One *cannot* write
        z =: (a,b) ;
("=:" is the assignment operator),
for z is just a macro, not a "pair" object or variable.

This can be *very* confusing when learning MetaPost.


        for i=0 upto n:
                z[i] = (i,0) ;
works as each element of the (pseudo)array z is unknown, at least the
first time that it is used.
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