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`getenv'--look up environment variable

     #include <stdlib.h>
     char *getenv(const char *NAME);
`getenv' searches the list of environment variable names and values
(using the global pointer "`char **environ'") for a variable whose name
matches the string at NAME.  If a variable name matches, `getenv'
returns a pointer to the associated value.
A pointer to the (string) value of the environment variable, or `NULL'
if there is no such environment variable.
`getenv' is ANSI, but the rules for properly forming names of
environment variables vary from one system to another.
   `getenv' requires a global pointer `environ'.