[R] how to use the function from another package

[Peter Yang]

Hi, I am trying to write a package(A) for myself and need to use a
function from another package(B) which is in R already(need to install
it before use). Could anyone tell me how to implement that? Also I
hope that my package gives an ERROR message(something like "STOP,
please install package B first") if the package B is not installed
yet.  It is my first time to write a package. Thanks very much for
your help.

Peter