2.19 if

The if-control structure takes the following form:

if ( <logical expression> ) then
  <conditional commands>
[elseif ( <logical expression>) then
  <conditional commands>]
[else
  <conditional commands>]
endif

The logical expressions may contain numerical comparisons with syntax:

<arithmetic expression> <operator> <arithmetic comparison>

The following operators are allowed:

.lt.    ! less than
.le.    ! less or equal
.gt.    ! greater than
.ge.    ! greater or equal
.eq.    ! equal
.ne.    ! not equal

Logical expressions can be combined by logical operators:

.not.   ! negation of the following expression
.and.   ! logical and
.xor.   ! logical exclusive or
.or.    ! logical or

Logical operations may be nested and grouped by brackets "(" and ")".