| special dict that automatically converts itself to human readable
  formula on str(). Implements += for convenient formula concatenation 
        
            | Methods |  |  
        |  |  
            |  | __add__ |  
        | 
__add__ ( self,  other )
 |  
            |  | __iadd__ |  
        | 
__iadd__ ( self,  other )
 |  
            |  | __init__ |  
        | 
__init__ ( self,  form=None )
 
        
            | Exceptions |  |  
        | ValueError, "some of the dictionary entries are not valid for formula_dict (%s => %s)" %(str( key ), str( val ) ) 
 |  |  
            |  | __mul__ |  
        | 
__mul__ ( self,  other )
 
        
            | Exceptions |  |  
        | TypeError, "formula_dict can be only multiplied by an integer" 
 |  |  
            |  | __str__ |  
        | 
__str__ ( self,  reverse=0 )
 |  
            |  | get_element_fraction |  
        | 
get_element_fraction ( self,  element )
 |  
            |  | get_html_repr_as_string |  
        | 
get_html_repr_as_string (
        self,
        outer_element=None,
        reverse=0,
        )
 |  
            |  | get_molecular_weight |  
        | 
get_molecular_weight ( self )
 |  
            |  | is_saturated_alkyl_chain |  
        | 
is_saturated_alkyl_chain ( self )
 |  
            |  | keys_in_order |  
        | 
keys_in_order ( self )
 |  
            |  | read_formula_string |  
        | 
read_formula_string ( self,  form )
 |  
            |  | sorted_keys |  
        | 
sorted_keys ( self )
 |  
            |  | to_tuple |  
        | 
to_tuple ( self )
 |  |