https://docs.mendix.com/refguide9/string-concatenation/
经典示例：
trim(
( if $ContactPerson/Firstname != empty and $ContactPerson/Firstname != ''
then $ContactPerson/Firstname + ' ' else '' ) +
( if $ContactPerson/Middlename != empty and $ContactPerson/Middlename != ''
then $ContactPerson/Middlename + ' ' else '' ) +
( if $ContactPerson/Lastname != empty and $ContactPerson/Lastname != ''
then $ContactPerson/Lastname + ' ' else '' ) +
( if $ContactPerson/Suffix != empty
then getCaption( $ContactPerson/Suffix )
else '')
)