public boolean isExternal() {
		return (isGlobal() || isWeak()) && getValue() == 0 && getSize() == 0 &&
			getType() == STT_NOTYPE &&
			getSectionHeaderIndex() == ElfSectionHeaderConstants.SHN_UNDEF;
	}
Returns true if this is an external symbol.
A symbol is considered external if it's 
binding is global and it's size is zero.
@return true if this is an external symbol
Ghidra/Features/Base/src/main/java/ghidra/app/util/bin/format/elf/ElfSymbol.java