/in/foo.c: In function 'UnionSet':
/in/foo.c:5:7: warning: implicit declaration of function 'Search' [-Wimplicit-function-declaration]
   if( Search(c,nc, a[i])==-1 )
       ^~~~~~
/in/foo.c: In function 'main':
/in/foo.c:18:2: warning: implicit declaration of function 'Output' [-Wimplicit-function-declaration]
  Output(a,na);
  ^~~~~~
/tmp/ccbTBB9s.o: In function `UnionSet':
foo.c:(.text+0x45): undefined reference to `Search'
foo.c:(.text+0x8c): undefined reference to `Search'
/tmp/ccbTBB9s.o: In function `main':
foo.c:(.text.startup+0xab): undefined reference to `Output'
foo.c:(.text.startup+0xba): undefined reference to `Output'
foo.c:(.text.startup+0xde): undefined reference to `Output'
collect2: error: ld returned 1 exit status