Definition of function base_yylex in version 8.1.4

Hi all,
I am having a project in Postgres where I need to modify some code in the parser section of the code base. During some analysis I came across a function called "base_yylex" which is called in the function yylex in the file parser.c. However, I was unable to find the definition of this function due to which I am stuck completely.
Looking forward to some help from experts.
Thanks in advance

Best regards,
Sayali

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