Get rid of unnecessary expr_yylex declaration (we haven't supported
flex 2.5.4 in a long time, and even if we still did, the declaration in
pgbench.h makes this one unnecessary and inappropriate). Fix copyright
dates, improve some layout choices, etc.
Previously, you could do \set variable operand1 operator operand2, but
nothing more complicated. Now, you can \set variable expression, which
makes it much simpler to do multi-step calculations here. This also
adds support for the modulo operator (%), with the same semantics as in
C.
Robert Haas and Fabien Coelho, reviewed by Álvaro Herrera and
Stephen Frost
