```typescript
export function lineTag(body: string): string {
  if (typeof body !== "string") {
    throw new Error("lineTag expects a string");
  }
  let sum = 0;
  for (const ch of body) {
    sum = (sum + ch.codePointAt(0)!) % 256;
  }
  return sum.toString(16).padStart(2, "0");
}

export function checkLine(line: string): string {
  if (typeof line !== "string" || line.length < 3) {
    throw new Error("checkLine expects a non-empty string with at least 3 characters");
  }
  const tildeIndex = line.lastIndexOf("~");
  if (tildeIndex === -1 || tildeIndex + 2 >= line.length) {
    throw new Error(
      "checkLine expects a line where the third-from-last character is a tilde"
    );
  }
  const body = line.slice(0, tildeIndex);
  const tag = lineTag(body);

  if (line.substring(tildeIndex + 1).trim() !== tag) {
    throw new Error("the last two characters of the line must match the computed tag");
  }

  return body;
}
```