d = '0123456789';   op = any('+-*/');   ws = ' ' char(9)
     sp1 = span(ws);  sp0 = sp1 | ''
     int = span(d)
     num = (any('+-') | '') int ('.' (int | '') | '')
     xpr = num . x sp1 num . y sp1 op . o (sp1 | rpos(0))
read line = input                                     :f(end)
     line pos(0) sp0 rpos(0)                          :s(read)
zapp line (pos(0) | notany(d)) . p '.' any(d) . q
+                              = p '0.' q             :s(zapp)
doop line sp0 xpr = ' ' eval(x + 0. ' ' o ' ' y) ' '  :s(doop)
     line pos(0) sp0 (num . output) sp0 rpos(0)       :s(read)
     output = 'error'                                 :(read)
end