def to_mixins_array(ast)
      v = ast.to_value
      mixins = v.is_a?(Hash) ? v.values : Array(v)
      mixins.all? {|mx| mx.is_a? String } ? mixins : []
    end
def to_mixins_array(ast)
      v = ast.to_value
      mixins = v.is_a?(Hash) ? v.values : Array(v)
      mixins.all? {|mx| mx.is_a? String } ? mixins : []
    end
converts AstNode, whether it's a string, array or hash into
 array of strings (when possible).