A piece of software closely matching the interface of a different piece of software doesn't automatically make the former based on the latter, except in an abstract sense (in which almost all ideas are derivative in some ways, because everything builds on the shoulders of giants). What is this, the CP/M vs MS-DOS argument again? I'm not Gates' biggest fan in terms of his ruthless business acumen, but claims of plagiarism, however slight, need a bit of hard evidence to be anything more than a smear.

If you look at the source code of a program, learn how it works, and then you make your own program with the same functionality, is your program "based on" the original, or is it independent work?