class SyntaxTree::Mermaid::FlowChart

def link(from, to, label = nil, type: :directed, color: nil)

Add a link to the flowchart between two nodes with an optional label. 
def link(from, to, label = nil, type: :directed, color: nil)
  link = Link.new(from, to, label, type, color)
  links << link
  output.puts("#{prefix}#{link.render}")
  link
end