[this is an alternate, separate system from "ConditionalBGA"]

Conditionals allow enabling BGAs or portions of BGAs based on a boolean
expression.

*  Quick start

To only show a BGA on Wednesday:

[BGAnimation]
Condition=Weekday() == 3

[Layer1]
File=star
Command=x,100;y,200
...


To only between 10pm and 3am, use this Condition= line:
   Condition=Hour() > 22 or Hour() < 3

Only if there are at least 2 stages left:
   Condition=NumStagesLeft() >= 2

Don't show in demo or jukebox mode:
   Condition=not IsDemonstration()

Only show if the current song is "Happy Birthday":
   Condition=CurSong() == Song("Happy Birthday")

Only display the layer on even days:
   math.mod(DayOfMonth(), 2) == 0

Only display the layer on odd days:
   math.mod(DayOfMonth(), 2) == 0

Only display on April Fools:
   Condition=MonthOfYear() == 4 and DayOfMonth() == 1

Only display on the final stage:
   Condition=IsFinalStage()

Only if the "reverse" modifier is in use by player 1:
   Condition=UsingModifier(1, "reverse")

Only if the "reverse" modifier is in use by player 1 or 2:
   Condition=UsingModifier(1, "reverse") or UsingModifier(2, "reverse")

Only if either player got an AAA or better:
   GetBestGrade() <= Grade("AAA") 

* Advanced use

This can be applied to a single layer of a BGAnimation, by putting
the conditional in the appropriate [Layer] section.

[Layer1]
File=stage1
Condition=StageIndex() == 0

[Layer2]
File=stage2
Condition=StageIndex() == 1

[Layer3]
File=extra_stage
Condition=IsExtraStage()

 * Complete function list:

MonthOfYear()    month of year (1..12)
DayOfMonth()     day of the month (1..31)
Hour()           hours past midnight (0..23)
Minute()         minutes after the hour (0..59)
Second()         seconds after the minute (0..59)
Year()           year (eg. 2004)
Weekday()        days since Sunday (0..6)
DayOfYear()      the number of days since January 1 (0..365)

StageStats info:

GetBestGrade()   best grade of any active player (AAAA is 0, AAA is 1, etc)
GetWorstGrade()  worst grade of any active player
OnePassed()      returns true if either player passed
FullCombo(pn)    returns true if player "pn" (0 or 1) got a full combo
MaxCombo(pn)     returns the given player's max combo
GetGrade(pn)     returns the given player's grade
Grade(grade)     returns the grade number for a given string
OneGotGrade(pn,n)