>>> help(Crossing)
Help on class Crossing in module regina.engine:

class Crossing(pybind11_builtins.pybind11_object)
 |  Represents a single crossing in a link diagram. The two strands of the
 |  link that run over and under the crossing respectively can be accessed
 |  through routines such as over(), under(), upper(), lower(), and
 |  strand().
 |  
 |  Each crossing has a sign, which is either positive (denoted by +1) or
 |  negative (denoted by -1):
 |  
 |  * In a positive crossing, the upper strand passes over the lower
 |    strand from left to right:
 |  
 |  ```
 |    -----\ /----->
 |          \
 |    -----/ \----->
 |  ```
 |  
 |  * In a negative crossing, the upper strand passes over the lower
 |    strand from right to left:
 |  
 |  ```
 |    -----\ /----->
 |          /
 |    -----/ \----->
 |  ```
 |  
 |  If a link has *n* crossings, then these are numbered 0,...,*n*-1. The
 |  number assigned to this crossing can be accessed by calling index().
 |  Note that crossings in a link may be reindexed when other crossings
 |  are added or removed - if you wish to track a particular crossing
 |  through such operations then you should use a pointer to the relevant
 |  Crossing object instead.
 |  
 |  ... (documentation continues) ...
