src/Composer/Autoload/ClassLoader.php
public function set($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr0 = (array) $paths;
        } else {
            $this->prefixesPsr0[$prefix[0]][$prefix] = (array) $paths;
        }
    }
public function set($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr0 = (array) $paths;
        } else {
            $this->prefixesPsr0[$prefix[0]][$prefix] = (array) $paths;
        }
    }
Registers a set of PSR-0 directories for a given prefix,
replacing any others previously set for this prefix.

@param string       $prefix The prefix
@param array|string $paths  The PSR-0 base directories