code: func (e *Error) ContextSource() []SourceLine {
  	if e.SourceLines == nil {
  		return nil
  	}
  	start := (e.Line - 1) - 5
  	if start < 0 {
  		start = 0
  	}
  	end := (e.Line - 1) + 5
  	if end > len(e.SourceLines) {
  		end = len(e.SourceLines)
  	}
  
  	lines := make([]SourceLine, end-start)
  	for i, src := range e.SourceLines[start:end] {
  		fileLine := start + i + 1
  		lines[i] = SourceLine{src, fileLine, fileLine == e.Line}
  	}
  	return lines
  }
docs: // ContextSource method returns a snippet of the source around
  // where the error occurred.