<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _construct_register(reg, default_reg):
 """Constructs a register dict."""

    if reg:
        x = dict((k, reg.get(k, d)) for k, d in default_reg.items())
    else:
        x = dict(default_reg)
    return x