Function declare_pipe_params 

pub fn declare_pipe_params<'a>(
    stmts: &[Stmt<'a>],
    arenas: Arenas<'a>,
    interner: &mut Interner,
) -> Vec<Stmt<'a>>

The program with every pipe parameter declared as one: ch: Real a pipe parameter becomes ch: Pipe of Real, so each site that types a parameter by its declaration sees the pipe.