Choose sentinel strings that are guaranteed not to appear in source.
The placeholders we splice into the masked source have the form
<open><digits><close>, so the restore pass must not confuse them
with content. Extends both sentinel characters until neither appears.
Choose sentinel strings that are guaranteed not to appear in
source. The placeholders we splice into the masked source have the form<open><digits><close>, so the restore pass must not confuse them with content. Extends both sentinel characters until neither appears.