BEGIN...END - Defines a block of statements.
IF...THEN...ELSE - Conditionally executes a statement or block of statements.
CASE - Selects one statement for execution, depending on the value of an expression.
SWITCH - Selects one statement for execution, depending upon the value of an expression.
FOR - Executes one or more statements repeatedly, incrementing or decrementing a variable with each repetition, until a condition is met.
REPEAT...UNTIL - Repeats statement(s) until expression evaluates to true. Subject is always executed at least once.
WHILE...DO - Performs statement(s) as long as expression evaluates to true. Subject is never executed if condition is initially false.
BREAK - Exits from a loop (FOR, WHILE, REPEAT), CASE, or SWITCH statement.
CONTINUE - Starts the next iteration of the enclosing FOR, WHILE, or REPEAT loop.
GOTO - Transfers program control to point specified by label.
COMPILE_OPT - Gives IDL compiler information that changes the default rules for compiling functions or procedures.
FORWARD_FUNCTION - Causes argument(s) to be interpreted as functions rather than variables (versions of IDL prior to 5.0 used parentheses to declare arrays).
FUNCTION - Defines a function.
PRO - Defines a procedure.
COMMON - Creates a common block.