Original sentence: In  the definition of 2 is called df-2 and it states that.It refers to the steps 1 and 2 of the theorem called 2p2e4 in.The rule of inference used to infer step from step 1 is opreq2i.This theorems states that  (it is a very well known mathematical result since Whithehead and Russell proved it in their famous Principia Mathematica).so Metamath uses substitution to associate the conclusion of opreq2i  with
Edited sentence: In "set.It refers to the steps 1 and 2 of the theorem called 2p2e4 in "set.The rule of inference used to infer step  from step 1 is opreq2i.This theorems states that  ( it is a very well known mathematical result since Whithehead and Russell proved it in their famous Principia Mathematica ).mm" the definition of 2 is called df-2 and it states that.opreq2i has an odd look and obviously it would never appear that way in a textbook.so Metamath uses substitution to associate conclusion of opreq2i  with