firstline – Print the first line of a paragraph in a different font

This sam­ple file de­fines a com­mand \first­line. Place the com­mand \first­line at the start of a para­graph, and it will set the first line sub­ject to the font com­mand \First­line­font.
The file is part of the Plain TeX sam­ples col­lec­tion.