foo.cpp: In function 'bool equal(double, double)':
foo.cpp:8:63: error: 'abs' was not declared in this scope
 inline bool equal(double lhs, double rhs) { return abs(lhs-rhs) < DELTA; }
                                                               ^
foo.cpp:8:74: warning: control reaches end of non-void function [-Wreturn-type]
 inline bool equal(double lhs, double rhs) { return abs(lhs-rhs) < DELTA; }
                                                                          ^