from typing import Dict, List, Set

class DependencyGraph:
    """Maintains cross-reference and dependency relationships."""
    
    def __init__(self):
        self.nodes: Set[str] = set()
        self.edges: Dict[str, List[str]] = {}
        
    def add_node(self, node: str):
        self.nodes.add(node)
        if node not in self.edges:
            self.edges[node] = []
            
    def add_edge(self, source: str, target: str):
        self.add_node(source)
        self.add_node(target)
        if target not in self.edges[source]:
            self.edges[source].append(target)
        
    def get_dependencies(self, node: str) -> List[str]:
        return self.edges.get(node, [])