<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def add_license(key, description, safety_checks=True,
                service_instance=None):
    '''
    Adds a license to the vCenter or ESXi host

    key
        License key.

    description
        License description added in as a label.

    safety_checks
        Specify whether to perform safety check or to skip the checks and try
        performing the required task

    service_instance
        Service instance (vim.ServiceInstance) of the vCenter/ESXi host.
        Default is None.

    .. code-block:: bash

        salt '*' vsphere.add_license key=<license_key> desc='License desc'
    '''
    log.trace('Adding license \'%s\'', key)
    salt.utils.vmware.add_license(service_instance, key, description)
    return True