Combine absolute path and relative path to get a new absolute path

I'm writing a program in which one of the components must be able to take a path it is given (such as /help/index.html, or /help/) and a relative path based on that location, (such as ../otherpage/index.html, or sub/dir/of/help/, or help2.html) and produce the absolute path implied by the relative path. Consider the following directory tree.

/
index.html
content.txt
help/
    help1.html
    help2.html

The file index.html contains a link like help/help1.html. The program is passed / or /index.html, and combines it with help/help1.html to get /help/help1.html.

Similarly, the file /help/help1.html has the link ../content.txt, from which the program needs to return /content.txt. Is there a reasonable way to do this?

Thank you.

Edit: Thank you to Stephen Weinberg! For everyone from the future, here's the code I used.

func join(source, target string) string {
    if path.IsAbs(target) {
        return target
    }
    return path.Join(path.Dir(source), target)
}

The path.Join when used with path.Dir should do what you want. See http://golang.org/pkg/path/#example_Join for an interactive example.

path.Join(path.Dir("/help/help1.html"), "../content.txt")

This will return /content.txt.

Stephen's answer is correct, but I wanted to add something to save future readers some time:

You should note functions within the path package assume the separator is /. When using the example above, I kept getting the output . since I had a Window's file path using \.

If you're not manipulating URLs, consider using the filepath package which uses the OS's directory separator.

E.g. When running on Windows:

path.Dir("C:\\Users\\Darren\\Desktop\\file.txt")
filepath.Dir("C:\\Users\\Darren\\Desktop\\file.txt")

Returns:

.
C:\Users\Darren\Desktop