${variable:=value}

Era Eriksson suggested some shell scripting for Jerry Shenk, saying,

| Oh, just set MAILDIR by hand before testing,
| and/or add something to the main script to set it to something
| reasonable if it's not already set. Hint: ${MAILDIR:=$HOME/Mail}

I just want to emphasize that Era was recommending that for the shell script,
not for the rcfile.  Procmail rcfiles understand ${variable-value},
${variable:-value}, ${variable+value}, and ${variable:+value}, but not
${variable=value} nor ${variable:=value}.