View Compilation Error

0_0_11753273_14714.cpp: In member function `bool interval::operator<(const interval&) const':
0_0_11753273_14714.cpp:27: error: `sgn' undeclared (first use this function)
0_0_11753273_14714.cpp:27: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_11753273_14714.cpp: In member function `bool node::operator<(const node&) const':
0_0_11753273_14714.cpp:36: error: `sgn' undeclared (first use this function)
0_0_11753273_14714.cpp: In function `int sgn(double)':
0_0_11753273_14714.cpp:48: error: `int sgn(double)' used prior to declaration