pwd: /out/package
ppid: 7129
   1: WRITELN echo 123 > 1.txt
   2: WRITELN echo 321 > 2.txt
   3: WRITELN echo abc < 1.txt < 2.txt
   4: WRITELN echo abc | grep abc < 1.txt
   5: WRITELN exit

mumsh $ mumsh $ mumsh $ error: duplicated input redirection
mumsh $ error: duplicated input redirection
mumsh $ exit

mumsh $ mumsh $ mumsh $ error: duplicated input redirection
mumsh $ error: duplicated input redirection
mumsh $ exit