[tex-k] Errors in The METAFONTbook?

[胡亚捷 (Hu Yajie)]

Page C83, lines 16 and 19
The book gives the computer's response as `0.5a=-0.5b-c+1.5' and `[d=]0.75b+
0.5c+0.75', but on my computer both MiKTeX and TeX Live (2017/Debian) respond
`0.5a=-c-0.5b+1.5' and `d=0.5c+0.75b+0.75' instead. Can someone confirm this?

Page C148, lines 8--9 & Page C214, lines 6--7
Change
    <future pen primary> ::= pencircle
        | makepen <path primary>
to
    <future pen primary> ::= <future pen argument>
        | pencircle
        | makepen <path primary>
by analogy to the syntax of <vacaous primary>. Hint: consider the program
    def foo secondary p = p yscaled 20 enddef;
    pickup foo pencircle xscaled 30; drawdot (0,0);
    pickup foo (pencircle xscaled 30); drawdot (100,0);